Some 95.5% of voters in Crimea have supported joining Russia, officials say, after half the votes have been counted in a disputed referendum.
Crimea's leader says he will apply to join Russia on Monday. Russia's Vladimir Putin has said he will respect the Crimean people's wishes.
Many Crimeans loyal to Kiev boycotted the referendum, and the EU and US condemned it as illegal.
Pro-Russian forces took control of Crimea in February.
They moved in after Ukraine's pro-Moscow president Viktor Yanukovych was ousted after street protests.
Mr Putin and US President Barack Obama spoke over the phone earlier, with the Kremlin and the White House later releasing contrasting accounts of the conversation.
The Kremlin said both men agreed to seek a way to stabilise Ukraine, and that Mr Putin had stressed that the Kiev government had failed to curb "rampant violence by ultra-nationalists".
The White House said Mr Obama had insisted that the referendum was illegal and would never be accepted, and called for Moscow to support an international monitoring mission in eastern Ukraine.
The EU said in a statement that the vote was "illegal and illegitimate and its outcome will not be recognised".
EU foreign ministers are due to meet on Monday and are expected to consider imposing sanctions on Russian officials.
Sergei Aksyonov, Crimea's leader installed last month after the Russian takeover, celebrated the referendum on stage in Simferopol.
Backed by the Russian national anthem, Russian flags, and the personnel of Russia's Black Sea fleet, he told supporters that Crimea was "going home".
Mr Aksyonov said Crimea's parliament, which was disbanded by the government in Kiev last week, would send a formal request to Moscow to join Russia on Monday.
Some 58% of people in Crimea are ethnic Russian, with the rest made up of Ukrainians and Tatars.
Most of the Tatars that the BBC spoke to said they had boycotted the vote, and felt that life under the Kremlin would be worse.
Refat Chubarov, leader of the Tatars' unofficial parliament, said the referendum was illegal, and held in a hasty manner under the control of Russian troops.
"The fate of our motherland cannot be decided in such a referendum under the shadows of the guns of soldiers," he told the BBC.
The Tatars were deported to Central Asia by Soviet dictator Joseph Stalin. They were only able to return with the fall of the Soviet Union and many want to remain in Ukraine.
But the referendum did not have an option for those who wanted the constitutional arrangements to remain unchanged.
Voters were asked whether they wanted to join Russia, or have greater autonomy within Ukraine.
Away from the Crimea region, unrest continued in the south-east Ukrainian city of Donetsk.
Pro-Russian protesters stormed the prosecutor's building shouting "Donetsk is a Russian city", and then broke into the local security services headquarters for the second time in two days.
They later dispersed but promised to return on Monday.