Officials from the airline said 298 people were on board the Boeing 777-200, which was apparently shot down over the war-torn country yesterday as it travelled from Amsterdam to Kuala Lumpur.
Among them were 154 Dutch passengers, 45 Malaysians, including 15 crew, 27 Australians, 12 Indonesians, four Germans, four Belgians, three Filipinos and one Canadian.
Three infants are among the dead and the nationalities of 41 passengers have yet to be verified. New Zealand authorities have also confirmed that one of its nationals died in the crash.
Around 100 of those killed were delegates on their way to an international conference on Aids in Melbourne.
The Australian newspaper reported that up to 108 researchers were aboard the flight heading for the conference.
News of the deaths of the Aids researchers and activists sparked an outpouring of grief across the global scientific community.
The World Health Organisation's Geneva-based spokesman Glenn Thomas, 49, understood to be a former BBC journalist who was travelling to the conference, was also among the dead, said Christian Lindmeier, spokesman for WHO's Western Pacific region.
"Everybody's devastated," Mr Lindmeier said. "It's a real blow."
Among the passengers was former president of the International Aids Society Joep Lange, a well-known HIV researcher from the Netherlands.
Chris Beyrer, president-elect of the International Aids Society, said if reports of Mr Lange's death were true, "then the HIV/Aids movement has truly lost a giant."
Nobel laureate Dr Francoise Barre-Sinoussi, co-discoverer of the Aids virus and president of the International Aids Society, paid tribute to Mr Lange in the Australian capital Canberra.
"Joep was a wonderful person - a great professional ... but more than that, a wonderful human being," she said. "If it is confirmed, it will be a terrible loss for all of us. I have no words, really, to try to express my sadness. I feel totally devastated."
She later told reporters the conference would continue out of respect for the lives lost "because we know that it's really what they would like us to do".
Mr Lange had been working on HIV since the earliest years of the epidemic, participating in clinical trials and research across the world, Dr Barre-Sinoussi said. He had dedicated his life, she said, to "the benefit of mankind".
Sharon Lewin, co-chairman of the conference, called Mr Lange a true renaissance man, who also had a keen interest in arts and literature.
"He was passionate about his job and passionate about global health and improving people's lives in low-income countries," she said. "He was quite visionary actually, I think since the very early days of the epidemic and could see what the challenges were that lay ahead."
Former US President Bill Clinton will deliver an address at next week's Aids conference, which brings together thousands of scientists and activists from around the world to discuss the latest developments in research.
Australian House of Representatives speaker Bronwyn Bishop called for a moment of silence in parliament to honour the victims. She will address the Aids conference on Monday.
The International Aids Society issued a statement expressing its grief over the news that several of its colleagues and friends were on board.
"At this incredibly sad and sensitive time the IAS stands with our international family and sends condolences to the loved ones of those who have been lost to this tragedy," the group said.
The plane was flying on a usual route that had earlier been declared safe by the International Civil Aviation Organisation, Malaysia Airlines said, while the International Air Transportation Association said the airspace it had been crossing was not subject to any restrictions.
All European flights operated by the airline will be taking alternative routes with immediate effect.
A spokesman for the airline said: "Malaysia Airlines is in the process of notifying the next of kin of the passengers and crew. Our focus now is to work with the emergency responders and authorities and mobilise its full support to provide all possible care to the next of kin.
"Our thoughts and prayers are with all affected passengers and crew and their family members."
Malaysia Airlines said it would release the passenger manifest of flight MH17 once all next of kin were notified.
It said on its website: "Our thoughts and prayers are with those on board flight MH17, as well as their families and loved ones."
Foreign Secretary Philip Hammond has called for an international investigation to establish what happened to Flight MH17 when it crashed into territory held by pro-Russian separatists.
Speaking at the Foreign Office following emergency talks with ministers and officials, Mr Hammond said they were still working through the passenger data to establish exactly how many Britons there were.
"I'm deeply shocked by this appalling incident and I send my heartfelt condolences to all those who may have lost family and friends. We're determined to get to the bottom of understanding what has happened here," he said.
"As yet we do not have any definitive information about how this incident occurred and I don't want to speculate at this stage. We believe that there must be a UN-led international investigation of the facts."
He said Britain was prepared to make Air Accident Investigation Branch assets and specialists available to assist an investigation.
The Government's emergency committee (Cobra) is expected to meet today to discuss the situation. The United Nations Security Council will hold an emergency meeting on Ukraine today, scheduled for 3pm British time.
US secretary of state John Kerry called for a credible investigation, saying: said: "We are horrified by the crash of Malaysia Airlines flight MH17. There are no words adequate to express our condolences to the families of the nearly 300 victims,
"We offer our sympathies and support to the governments of Malaysia and the Netherlands at this difficult time, as well as to all those whose citizens may have been on board. We are reviewing whether any American citizens were aboard the flight.
"The United States government remains prepared to assist with a credible, international investigation any way we can, and we will continue to be in touch with all relevant partners as we seek the facts of what happened today."
US vice president Joe Biden said, however, it did appear the aircraft had been shot down - while US officials said their intelligence analysis showed it had been hit by a surface-to-air missile.
"I say 'apparently' because we don't actually have all the details. I want to be sure of what I say. Apparently, have been shot down. Shot down, not an accident. Blown out of the sky," Mr Biden said.
His position was echoed by Australian prime minister Tony Abbott, who condemned the downing of flight MH17 as "a grim day for our country...and a grim day for our world" during an address to the country's parliament, The Sydney Morning Herald reported.
Suggesting the plane had been shot down "it seems by Russian-backed rebels", he said that "as it stands, this looks less like an accident, than a crime".
He added: "Let me conclude with this: the bullying of small countries by big ones, the trampling of justice and decency in the pursuit of national aggrandisement and reckless indifference to human life should have no place in our world."
Mr Abbott also told radio station Fairfax Radio 3AW that if the plane had been shot down by a missile supplied by Russia, then ''Russia bears a heavy share of responsibility'' for the disaster. He called for the perpetrators to "swiftly be brought to justice".
Ukraine's UN ambassador Yuriy Sergeyev tweeted after the crash that "Ukraine will present the evidence of Russian military involvement into the Boeing crash. This crime should be fully investigated".
The disappearance of the airliner from the radar screens as it flew over eastern Ukraine prompted immediate allegations and counter-claims, with the government in Kiev and the separatist rebels blaming each other.
The rebels insisted that they did not have the equipment capable of bringing down an aircraft which was flying at such high altitude at more than 30,000 feet.
But Ukraine's security services produced what they said were two intercepted telephone conversations that they said showed rebels were responsible.
In the first call, the security services said, rebel commander Igor Bezler told a Russian military intelligence officer that rebel forces shot down a plane yesterday.
In the second, two rebel fighters - one at the scene of the crash - said the rocket attack was carried out by a unit of insurgents about 15 miles north of the crash site.
Graphic images from the crash scene near the village of Grabovo showed a pall of thick smoke hanging above the site and bodies among the charred wreckage.
Malaysian prime minister Najib Razak said "no stone can be left unturned" in the effort to find out what happened, and said that the plane did not make any distress call.
"If it transpires that the plane was indeed shot down, we insist that the perpetrators must swiftly be brought to justice," he said.
He said the Ukrainian authorities had agreed to open negotiations with the rebels to create a safe corridor to the crash site.
Prime Minister David Cameron said he was "shocked and saddened" by the tragedy, while Ukrainian president Petro Poroshenko described it as a "terrorist act".
Ukraine has been torn apart by internal strife since the overthrow of the Moscow-backed regime of Viktor Yanukovych, with Russian backed separatists already accused by the authorities in Kiev of shooting down military jets with missiles supplied by Russia.
The Ukrainian government immediately blamed the separatists with President Poroshenko declaring the "armed forces of Ukraine did not take action against any airborne targets".
Anton Gerashenko, an aide to the Ukrainian interior minister, said on his Facebook page that the plane was flying at an altitude of 33,000 feet when it was hit by a missile fired from a Buk launcher, a Soviet era surface-to-air missile system capable of taking down a high altitude aircraft.
A similar launcher was reportedly seen by journalists near the eastern Ukrainian town of Snizhne earlier yesterday.
But a spokesman for the rebels said the plane must have been shot down by Ukrainian government troops while Russian president Vladimir Putin laid the blame for the crash on Ukraine.
"This tragedy would not have happened if there were peace on this land, if the military actions had not been renewed in south-east Ukraine. And, certainly, the state over whose territory this occurred bears responsibility for this awful tragedy," he said.
The incident will prompt questions as to why the route over Ukraine was still being used, given previous incidents.
A UK Civil Aviation Authority (CAA) spokesman said: "The Ukrainian authorities are responsible for managing their airspace and the UK or other countries cannot enforce airspace restrictions in the area.
"However, the CAA has previously issued advice to UK airlines on operating in this area and following this incident, Eurocontrol (the European air traffic control body) has issued advice to airlines to plan routes that avoid the area."
The Department for Transport in London said flights were now routed around the area by air traffic control in the region.
"Pilots around the world have been advised to plan routes that avoid the area by Eurocontrol, the European organisation for the safety of air navigation," a DfT spokesman said.
On Wednesday night the Ukrainian authorities said one of their fighter jets was shot down by an air-to-air missile from a Russian plane and Ukrainian troops were fired upon by missiles from a village inside Russia.
The episodes add to what Ukraine says is mounting evidence that Moscow is directly supporting separatist insurgents in eastern Ukraine who have substantial quantities of powerful weapons.
Ukraine said a military transport plane was shot down on Monday by a missile fired from Russian territory. Security Service chief Valentyn Nalyvaichenko said he had "unconditional evidence" that Russia was involved in downing that aircraft.
The incident brings tragedy to Malaysia Airlines for the second time this year.
In March, one of its jets disappeared with 227 passengers and 12 crew on board in one of the greatest aviation mysteries of all time.
It would not be the first time a civilian airliner has been mistakenly shot down.
In 1988, an Iran Air flight from Tehran to Dubai was shot down by the US warship USS Vincennes in the Persian Gulf. All 290 on board, including 66 children and 16 crew, died.
In 1983, Korean Air Lines Flight 007 from New York to Seoul via Anchorage was shot down by a Soviet military jet near Sakhalin Island in the East Sea. All 269 passengers and crew were killed. The Soviets initially denied knowledge of the incident but later admitted responsibility, claiming that the aircraft was on a spy mission.
A Labour MP called for the House of Commons to delay its summer recess, which is due to begin on Tuesday, if it is found that the missile used to down the aircraft was supplied by the Russian government.
Bassetlaw MP John Mann said: "This is a very troubling development which raises serious issues that Parliament will want to consider. It cannot do this if it is not sitting."