MOSCOW – Ukraine plunged further into crisis yesterday as the Western-leaning opposition leader, Viktor Yushchenko, claimed the presidency and tens of thousands of his supporters marched to the presidential office in defiance of poll results they say were rigged by the pro-Moscow government.

“Ukraine is on the threshold of a civil conflict,” Mr. Yushchenko warned lawmakers gathered for an emergency session. There were no reports of violence as demonstrators faced off with hundreds of riot police gathered around the presidential administration building.

“We are going to the presidential administration in a peaceful way, without breaking anything,” Mr. Yushchenko’s firebrand lieutenant, Yulia Tymoshenko, told the crowd on its way to the building. “And either they give up their power, or we will take it.” Mr. Yushchenko called on police to cross over and join the opposition, saying “Ukraine needs you, come over to our side.”

The outgoing president, Leonid Kuchma, called for negotiations among all sides in Ukraine’s spiraling political crisis today, hours after the opposition leader declared himself the winner of a disputed presidential election to the approval of tens of thousands of street protesters. A top opposition figure accepted the outgoing president’s proposal, the Interfax news agency reported.

Despite freezing temperatures and blowing winds, the number of protesters on the streets of Kiev continued to increase since the demonstration began early Monday. Their ranks were bolstered by opposition supporters arriving from across the country and the protesters numbered more than 200,000 last night. Downtown Kiev was transformed into a sea of orange – the opposition’s trademark color. Many also waved the red and white flag of Georgia, which this time last year saw similar protests that toppled the government and led to the election of a pro-Western president, Mikhail Saakashvilli.

The opposition accuses authorities of rigging Sunday’s vote in favor of the prime minister, Viktor Yanukovich. Official ballot counts showed Mr. Yanukovich won 49.39% of the vote compared with Mr. Yushchenko’s 46.71%, while exit polls showed Mr. Yushchenko leading by as many as 11 percentage points.

Five Ukrainian cities – including Kiev – have refused to recognize the results. But in a sign of the deep divisions in Ukraine, the regional government in Donetsk, Mr. Yanukovich’s hometown, yesterday denounced the protests in Kiev and called on the opposition “to accept defeat with dignity and to accept the complete and irrevocable victory of Yanukovich.”

The election was seen as a crucial test of whether the ex-Soviet state of 48 million will tilt westward or remain in the Kremlin’s sphere of influence. Mr. Yushchenko campaigned on a pro-market, liberal platform, promising to bring Ukraine into NATO and the European Union, while Mr. Yanukovich pledged closer ties with traditional ally Russia, which openly supported him.

Mr. Yushchenko yesterday made the dramatic gesture of taking the oath of office in Ukraine’s Parliament. During an emergency session called by the opposition to hold a no-confidence vote in the government, Mr. Yushchenko, wearing an orange tie, read the oath with his hand on the Constitution and a Bible. Supporters sang the national anthem and chanted “Bravo, Mr. President.”

However, the speaker of Parliament, Volodymyr Lytvyn, called the move a “political act” that “cannot entail any legal consequences.” The no-confidence failed after fewer than the required number of lawmakers showed up to vote.

In a statement, Mr. Yushchenko called on “Parliaments and people of the world” to recognize him as the real winner. “We are facing a threat to the will of a people of one of Europe’s largest countries,” he said.

President Putin called for a peaceful resolution of the crisis and warned Western governments against rushing to support the opposition.

“We can’t recognize or protest because no official results have yet been announced,” he said during a visit to Lisbon. “I can advise others to follow our example.”

Western governments have declared the vote – a run-off required after neither candidate won a majority in the October 31 first-round election – as seriously flawed and warned of consequences if the Ukrainian government recognizes its results.

Speaking to reporters in Berlin yesterday, the Republican chairman of the Senate Foreign Relations Committee, Richard Lugar, said Mr. Kuchma had already notched up “two strikes” by failing to ensure the two rounds of voting were free and fair.

He said it’s now time for Mr. Kuchma to face up to his responsibilities.

“It’s conceivable the president may not step up to it, he may finally say this is a bridge too far. …That’s sort of strike three, and there will be consequences, a lot of them.”

Washington has warned that it could review its relationship with Ukraine and take measures against those responsible for electoral fraud, such as freezing their assets in America or refusing to grant them travel rights.