Investigators today pledged to "go to the ends of the earth" to find whoever was behind the Boston Marathon bombings that took the lives of three people, including an eight-year-old boy.

Martin Richard was among those killed in the devastating blasts as he waited with his mother and sister to watch his father cross the marathon finish line.

President Barack Obama confirmed the FBI was investigating the tragedy as an act of terrorism, as it emerged that some 170 people were injured in the explosions.

Meanwhile, London Marathon organisers said the event will go ahead on Sunday as planned with the support of the Metropolitan Police and the city's mayor Boris Johnson.

And Scotland Yard's most senior police officer said security around Baroness Thatcher's funeral and the London Marathon will be stepped up in the next few days.

Richard DesLauriers, the special agent in charge of the Boston FBI office, told a press conference: "This will be a worldwide investigation. We will go to the ends of the earth to find the subject or subjects responsible for this despicable crime."

A candle has been reportedly placed on the doorstep of Martin Richard's home, while the word "Peace" has been written in chalk on the front path.

Betty Delorey, who lives near Martin's family home, told reporters Martin loved to climb the neighbourhood trees and hop over the fence outside his home.

His father, Bill, is the director of a local community group, while his mother Denise works at the Neighbourhood House Charter School.

Addressing a press conference, Mr Obama said "This was a heinous and cowardly act. The FBI is now investigating this as an act of terrorism."

But he added: "The American people refused to be terrorised. What the world saw yesterday in the aftermath of the explosions were stories of heroism and kindness and generosity and love."

There were no immediate claims of responsibility for the attack, the most serious in the US since the 9/11 atrocity.

The Pakistani Taliban, which has threatened attacks in the US because of its support for the Pakistani government, denied any role in the bombings.

Police were reportedly searching a flat in the Boston suburb of Revere as part of the investigation, which local media claim is home to a Saudi national injured in the blast.

Massachusetts State Police confirmed that a search warrant was served last night but gave no more details.

Mr DesLauriers said: "Our mission is clear - to bring to justice those responsible for the marathon bombing.

He added: "This remains a very active investigation, our ongoing investigation in various locations throughout the area goes on.

"However, there are no known additional threats. We continue to interview various witnesses and process the crime scene, which could take some time."

He said the FBI had received "voluminous" tip-offs over the last 18 hours.

The Boston Marathon is one of the world's oldest and most prestigious and about 23,000 runners took part, including hundreds from the UK and Ireland.

Scores of victims were hurt, many suffering shrapnel wounds, in the two explosions, which were caused by bombs believed to be stashed in bins nearby.

It is also understood the explosive devices were made of pressure cookers packed with metal and ball bearings.

Massachusetts General Hospital said "several amputations" had to be performed on survivors.

Horrifying images of injured spectators and blood-spattered pavements were caught as television cameras broadcast footage of the explosions.

Tim Davey, of Richmond, Virginia, was with his wife, Lisa, and children near a medical tent that had been set up to care for fatigued runners when the injured began arriving.

"They just started bringing people in with no limbs," he said.

The first explosion occurred just before the photo bridge that marks the finish line. The second could be heard a few seconds later.

They happened about four hours into the race, a time when large numbers of runners would be finishing, with many more relatives and friends cheering them on.

Confirming the London Marathon will go ahead, London Marathon chief executive Nick Bitel said: "We want to reassure our runners, spectators, volunteers and everyone connected with the event, that we are doing everything to ensure their safety and that the Virgin London Marathon 2013 is an outstanding success."

Sports minister Hugh Robertson insisted he was "absolutely confident" that the London event could be kept safe.

He said: "I think this is one of those incidents where the best way to show solidarity with Boston is to continue and send a very clear message to those responsible."

Met Police commissioner Sir Bernard Hogan-Howe said: "We are reviewing our plans which is sensible, but our reassurance is that we will keep people in London safe.

"We will increase searching, we will make sure we've got more officers on the street looking after people, making sure they're safe.

"We've no reason to think they're any less safe than they were before the terrible events in Boston yesterday."

A spokeswoman for the Home Office confirmed that Home Secretary Theresa May had been fully briefed and held a meeting with MI5 director-general Jonathan Evans and the Met's assistant commissioner for specialist operations, Cressida Dick.

Counter-terrorism expert Richard Barrett said the incident had hints of a right-wing attack rather than al Qaida-inspired extremism.

Mr Barrett, the former United Nations co-ordinator for the al Qaida and Taliban monitoring team, said the timing on Patriots' Day and the relatively small size of the devices suggested the work of a domestic extremist.

Mr Barrett, who has served with MI5 and MI6, said: "At the moment it looks more likely that it was a right-wing terrorist incident, rather than an al Qaida attack because of the size of the devices."

He added: "This happened on Patriots' Day, it is also the day Americans are supposed to have their taxes in, and Boston is quite a symbolic city. These are all little indicators."