US President Barack Obama has told his Russian counterpart Vladimir Putin that Russia has flouted international law by sending troops to Ukraine.
In a 90-minute telephone conversation, Mr Obama urged the Russian leader to pull forces back to bases in Crimea.
Mr Putin responded by saying that Moscow reserves the right to protect its interests and those of Russian speakers in Ukraine, the Kremlin said.
Meanwhile, Canada has recalled its ambassador to Moscow for consultations.
Canadian Prime Minister Stephen Harper said he was also suspending Canada’s preparations for a G8 summit in the Russian resort of Sochi in June.
Ukraine says it has put its army on full combat alert after Russia’s parliament approved the deployment of Russian troops.
Acting President Olexander Turchynov said he had also stepped up security at key sites, including nuclear plants.
Mr Obama, the White House said, told Mr Putin that the appropriate way to address any concerns “is peacefully through direct engagement” with the Ukrainian government and international mediating bodies.
“President Obama expressed his deep concern over Russia’s clear violation of Ukrainian sovereignty and territorial integrity,” the White House said.
Mr Obama told Mr Putin his actions were a “breach of international law, including Russia’s obligations under the UN Charter, and of its 1997 military basing agreement with Ukraine”, a statement added.
The Kremlin said that in his phone call with Mr Obama, President Putin “underlined that there are real threats to the life and health of Russian citizens and compatriots on Ukrainian territory”.
As diplomatic efforts increased, US Secretary of State John Kerry said he had spoken with foreign ministers from Europe and Canada as well as EU foreign policy chief Catherine Ashton and the Japanese envoy to the US “to co-ordinate on next steps”.
UN Secretary General Ban Ki-moon has called for “an immediate restoration of calm and direct dialogue”, whilst Nato chief Anders Fogh Rasmussen tweeted: “Urgent need for de-escalation in Crimea.”
The UN Security Council was holding an emergency session on the crisis on Saturday, and Nato and EU officials are due to hold talks in the coming days.
In his live television address, President Turchynov urged Ukrainians to bridge divisions in the country and said they must not fall for provocations.
He was accompanied by acting Prime Minister Arseniy Yatsenyuk, who said he was “convinced” Russia would not intervene militarily “as this would be the beginning of war and the end of all relations”.
But tensions are high, not only in Crimea which is home to a large number of ethnic Russians.
There were big pro-Russian rallies in several Ukrainian cities on Saturday.
In Donetsk, traditional stronghold of ousted Ukraine President Viktor Yanukovych, demonstrators from a crowd of some 7,000 tried unsuccessfully to occupy the regional administration building, raising a Russian tricolour on a nearby flag-post.
In Kharkiv, Ukraine’s second-biggest city, dozens of people were injured after scuffles between pro- and anti-Russian protesters broke out outside the regional administration building.
In Mariupol, in the south-east, hundreds of protesters carrying Russian flags gathered outside the city council.
President Putin submitted the request for troops “in connection with the extraordinary situation in Ukraine and the threat to the lives of Russian citizens”, the Kremlin said.
The upper house went into a special session almost immediately after Mr Putin made the request, and swiftly approved it.
Deputy Foreign Minister Grigory Karasin said, however, that this “does not mean that this right will be used quickly” to deploy troops.
During the upper house debate, one legislator accused President Obama of crossing “a red line” with his comments on Friday that there would be “costs” if Russia intervened militarily in Ukraine.
The upper house has recommended that the Russian ambassador to the US should be recalled, although the decision lies with Mr Putin.
Despite the political developments on Saturday, observers have been watching a build up of Russian military activity in Crimea – home to Russia’s Black Sea Fleet – for the past few days.
Latest reports say two Russian anti-submarine warships have appeared off the coast of Crimea in violation of an agreement governing the presence of Russia’s Fleet in the peninsula.
Russian soldiers are widely reported to be guarding a number of administrative buildings and military bases in Crimea. Parliament, airports, the state television building and telecommunications hubs have also been surrounded.
Some 6,000 extra Russian troops and 30 additional armoured vehicles are now in Crimea, Ukrainian Defence Minister Ihor Tenyukh said earlier on Saturday.
The newly-elected pro-Moscow leader of Crimea, Sergiy Aksyonov, said he had appealed to Mr Putin for help to ensure peace on the peninsula.
The interim government in Kiev does not recognise Mr Aksyonov and his government, and signed a decree on Saturday that their election at an emergency session of the regional parliament this week was illegal.