David Cameron has told Moscow there is "no excuse" for military intervention in Ukraine as he reiterated warnings that the "world is watching".
Britain called an emergency meeting of the UN Security Council after Russia authorised military action on Ukrainian soil, a move which is being viewed with "growing concern".
Foreign Secretary William Hague has summoned the Russian ambassador to the Foreign Office to hear the UK's reaction as the crisis deepened.
A pro-Russian supporter waves a Russian flag in front of pro-Russian armed men in military fatigues blocking the base of the Ukrainian frontier guards
Cameron said: "Britain views the developments in the Ukraine with growing concern.
"This afternoon, in view of the seriousness of events unfolding there today, and the Russian Parliament's decision to authorise Russian military intervention, the United Kingdom has called an emergency meeting of the UN Security Council.
"There can be no excuse for outside military intervention in Ukraine - a point I made to President Putin when we spoke yesterday.
"Everyone must think carefully about their actions and work to lower, not escalate tensions. The world is watching."
Foreign Secretary William Hague said earlier he was "deeply concerned" at the decision of the Russian parliament to back Vladimir Putin's call for the use of troops in Ukraine to protect ethnic Russians and a military base in Crimea.
"This action is a potentially grave threat to the sovereignty, independence and territorial integrity of Ukraine," Mr Hague said.
Hague, who is due to have discussions in Kiev tomorrow with interim leaders, talked by telephone to his Russian counterpart Sergey Lavrov today to call for calm as tensions heighten over the strategic region.
Ukraine has accused Moscow of a "military invasion" after armed men yesterday seized control of airports in Crimea, but the pro-Russian leader of the region today called on Moscow for support.
President Putin said Russia's military needed to be used "until the normalisation of the political situation in that country".
Hague said: "I am deeply concerned at the escalation of tensions in Ukraine, and the decision of the Russian parliament to authorise military action on Ukrainian soil against the wishes of the Ukrainian government. This action is a potentially grave threat to the sovereignty, independence and territorial integrity of Ukraine. We condemn any act of aggression against Ukraine.
"I spoke today to Russian foreign minister Lavrov to urge steps to calm this dangerous situation. I told minister Lavrov that Britain supports the Ukrainian government's request for urgent consultations in accordance with the 1994 Budapest Memorandum signed by the UK, US, Russia and Ukraine. In the light of President Putin's request to the Federation Council, we have now summoned the Russian Ambassador to the Foreign Office to register our deep concerns."
He added: "Yesterday I spoke to Ukrainian Acting President Turchynov and made clear the UK's support for Ukraine's new government. I urged him to ensure that the government takes measures which unify the country, and that it protects the rights of all Ukraine's citizens, including those from minority groups, in a spirit of inclusiveness.
"And I assured him of the UK's commitment to working with other international partners and institutions to ensure that reforms by Ukraine are matched by international willingness to provide economic support.
"I will visit Ukraine on Sunday to discuss these issues directly with the Ukrainian government. I will reiterate the UK's support for the territorial integrity of Ukraine. I will also discuss how the UK can support the Ukrainian government in recovering improperly acquired assets.
"The EU must agree urgently an asset freezing regime to target those suspected of laundering the proceeds of corruption. On my instructions, the British Embassy in Kiev has told the Ukrainian government that we stand ready to provide Ukraine with technical advice on asset recovery."
Shadow foreign secretary Douglas Alexander said: "The vote in the Russian parliament to approve Russian forces being mobilised is deeply concerning and represents a potentially grave escalation of the situation in Ukraine.
"In the critical hours and days ahead all efforts must be made to try to secure a diplomatic resolution to this crisis.
"The EU Foreign Affairs Council should convene in Brussels without delay, alongside the emergency meeting of the UN Security Council, to ensure the international community is united in its support for the territorial integrity of Ukraine and the will of its citizens.
"The Ukrainian government has called for urgent negotiations to resolve the crisis, in line with the terms of the 1994 Budapest Memorandum to which Russia is a signatory. The UK Government must now urge President Putin to accept that call for dialogue before taking any further action."
The unanimous vote by the Russian parliament formalised what Ukrainian officials described as the deployment of Russian troops in the strategic region of Crimea.
It also raised the possibility that Moscow could send its military elsewhere in Ukraine.