Russian President Vladimir Putin yesterday demanded and won his parliament's authority to invade Ukraine.
Moscow's move has set alarm bells ringing in Washington and London, creating the biggest direct confrontation between Russia and the West since the Cold War.
Putin's open assertion of the right to deploy troops in a country of 46 million people on the ramparts of central Europe follows days of warnings from US President Barack Obama and other Western leaders that Russia must not intervene, and assurances from Moscow that it would not do so.
The latest escalation in the crisis followed in the wake of troops seizing control of the Crimea peninsula and pro-Moscow demonstrators hoisting flags above government buildings in two eastern cities over the last few days.
Yesterday, Putin swiftly secured unanimous approval from Russia's senate to approve force "in connection with the extraordinary situation in Ukraine, the threat to the lives of citizens of the Russian Federation, our compatriots" and to protect the Black Sea Fleet in Crimea. The authorisation to use force in Ukraine would last "until the normalisation of the socio-political situation in that country".
The upper house swiftly delivered
a unanimous yes vote, shown on live television. Following the vote, tensions rose dramatically and scores of people were hurt in clashes between pro-Russian demonstrators and supporters of Kiev's new authorities in eastern cities near the Russian frontier.
Though ethnically Ukrainian, most people there speak Russian, and many support deposed President Viktor Yanukovich and Moscow.
By nightfall, demonstrators had torn down Ukrainian flags and replaced them with Russian flags on government buildings in the cities of Kharkiv, Donetsk, Odessa and Dnipropetrovsk.
In Kharkiv, scores of people were wounded in violent clashes when thousands of pro-Russian activists stormed the regional government headquarters and fought with a smaller number of supporters of Ukraine's new authorities.
Pro-Russian demonstrators wielded axe handles and chains against those defending the building with plastic shields.
In Donetsk, Yanukovich's home region, lawmakers declared they were seeking a referendum on the region's status.
"We do not recognise the authorities in Kiev, they are not legitimate," protest leader Pavel Guberev thundered from a podium in Donetsk.
The rapid pace of events has rattled Ukraine's new leaders, who took power of a nation on the verge of bankruptcy when Yanukovich fled Kiev last week after his police killed scores of anti-Russian protesters in Kiev.
After Russia's announcement of its intervention, Ukraine's acting president, Oleksander Turchynov, called a meeting of his security chiefs. Vitaly Klitschko, another anti-Yanukovich leader, called for general mobilisation.
After a meeting with security and defence chiefs lasting more than three hours yesterday, Turchynov put Ukraine's armed forces on full combat alert.
Prime Minister Arseny Yatseniuk said he had urged Russia to return its troops to their Crimea base during a phone call with Prime Minister Dmitry Medvedev and called for talks.
"Military intervention would be the beginning of war and the end of any relations between Ukraine and Russia," Yatseniuk told reporters.
In Kiev's Independence Square, where protesters camped for months against Yanukovich, a world war two film about Crimea was being shown on a giant screen, when Yuri Lutsenko, a former interior minister, interrupted it to announce: "War has arrived."
Hundreds of people descended on the square chanting: "Glory to the heroes. Death to the occupiers."
For many in Ukraine, the prospect of military conflict chilled the blood.
"When a Slav fights another Slav, the result is devastating," said Natalia Kuharchuk, a Kiev accountant. "God save us."
On an international diplomatic level the concern was also palpable.
British Foreign Secretary William Hague, who travels to Ukraine today for talks with the new leadership, urged his Russian counterpart to act to ease tensions. Hague, who has also summoned the Russian ambassador to the Foreign Office, said he was "deeply concerned" at the decision of the Russian parliament to back President Putin's call for the use of troops.
"This action is a potentially grave threat to the sovereignty, independence and territorial integrity of Ukraine," he said.
Prime Minister David Cameron said Britain had called an emergency meeting of the UN Security Council in view of the "serious and concerning events" in Ukraine.
"There can be no excuse for outside military intervention in Ukraine - a point I made to President Putin when we spoke yesterday (Friday)," Cameron said.
"Everyone must think carefully about their actions and work to lower, not escalate, tension. The world is watching."
Cameron's words were earlier echoed by President Barack Obama who said on Friday that "any violation of Ukraine sovereignty... would be deeply destabilising".
He warned of the "costs" of any Russian intervention in the Ukraine and commended the interim government in Kiev for its "restraint".
But yesterday, Senator John McCain, a leading voice on foreign affairs who often advocates a more forceful approach by Washington, called on Obama to "articulate exactly what the costs" will be.
"Every moment that the United States and our allies fail to respond sends the signal to President Putin that he can be even more ambitious and aggressive in his military intervention in Ukraine," Republican McCain said in a statement.
Late last, night President Obama's National Security Team was reported to have met for an update on the situation and to discuss its potential policy options in response.