Prime Minister Viktor Yanukovich was officially declared Monday as winner of the Ukrainian presidential election, but the opposition said there was blatant fraud in the vote and called for a general strike.
Ukraine's Central Election Commission said Yanukovich won 49.46 percent of the vote, and his rival, Viktor Yushchenko, 46.61 percent.
After the election commission announcement, Yanukovich pledged to "improve our lives and we will do it together."
"Tomorrow, we start talks with Yushchenko's team. We will look for common ground."
Yushchenko, however, refused to concede, citing brazen fraud and calling for a nationwide strike.
"People will have to look for truth in the streets in open struggle," he said. "(A strike will be) our answer to the lawlessness of Kuchma and Yanukovich."
Observers said the call risked triggering a crackdown by the government of outgoing President Leonid Kuchma, which has described the opposition's mass protest in Kiev as a prelude to a coup d'etat.
After the Commission's announcement, Kuchma warned of a civil war and asked for restraint from the two camps.
He called on the world community to "refrain from interfering in Ukraine's affairs," clearly in response to comments of the United States and Europe that the election was not up to international standard.
Tens of thousands of supporters of Yushchenko have rallied in central Independence Square in Kiev for the third day, demanding the overturn of poll results and the declaration of Yushchenko's victory.
Yushchenko called on his followers to go on strike throughout the country.
His major ally, Oleksandr Moroz, said: "We are organizing citizens, stopping lessons at schools and universities, stopping work at enterprises, stopping transport... we'll force the authorities to think about what they are doing."
The election controversy is likely to further divide the country.
Yushchenko enjoys strong backing in the west, a traditional stronghold of nationalism, while Yanukovich draws much support from the pro-Russian, heavily industrialized eastern half of the country.
As a former republic of the collapsed Soviet Union, Ukraine, with a population of 50 million, inherited from the Union many factories and arsenals including nuclear warheads.

WEST, RUSSIA CLASH OVER UKRAINE
The rift over the poll between Russia and the West, who prefers Yushchenko, has also grown.
Russian President Vladimir Putin has already expressed his congratulation to Yanukovich on his victory, and the Russian parliament slammed the Ukrainian opposition for its "illegal actions."
Outgoing US Secretary of State Colin Powell said the United States cannot accept the results as legitimate and called for a full review.
The election "does not meet international standards," and " there has not been an investigation of the numerous and credible reports of fraud and abuse," Powell said.
He said the Ukrainian authorities didn't demonstrate "their commitment to democracy and failed to be "a model for the region and the world."
UN chief Kofi Annan also voiced his concern. His spokesman Fred Eckhard said: "The Secretary-General believes a credible electoral process is critical to overcoming the current impasse."
Annan called on all sides to exercise maximum restraint and to adhere to democratic principles.
Canadian Deputy Prime Minister Anne McLellan said her government "cannot accept that the announced results... reflect the true democratic will of the Ukrainian people."
European Commission President Jose Manuel Barroso warned if Ukraine does not take a "serious, objective review," its relations with the EU would be affected.
The Netherlands, which holds the rotating EU presidency, would send a special envoy, Niek Biegman, to Ukraine to meet Yushchenko and the head of the constitutional court, which has the right to declare the vote invalid.