Thousands of demonstrators have laid siege to government buildings in the Ukrainian capital, Kiev, in protest at the presidential election results.
Ranks of police have been seen joining the opposition
Support for opposition leader Viktor Yushchenko appears to be growing as police cadets and TV stations openly show their allegiance, say observers.
Outgoing president Leonid Kuchma urged "this so-called revolution" to end.
He will meet Mr Yushchenko and Prime Minister Viktor Yanukovych - winner of the disputed poll - later on Friday.
EU foreign policy chief Javier Solana, Polish President Aleksander Kwasniewski and Lithuanian President Valdas Adamkus will be present, said Mr Solana's spokeswoman.
Ukraine's Supreme Court has suspended the presidential poll result while it considers the opposition's complaints.
According to the official election result, the pro-Russian Mr Yanukovych won with 49.46% of the vote against Mr Yushchenko's 46.61%.
But supporters of Mr Yushchenko, who favours closer ties with the EU, say the authorities oversaw massive fraud and independent observers reported widespread abuses.
Hundreds of thousands of protesters are gathered in central Kiev, waving banners and singing after spending their fifth consecutive night on the streets.
Thousands more protesters have surrounded nearby government buildings, including the residence of President Kuchma, aiming to paralyse government business.
Yanukovych (left): 49.46%
Western observers report:
Abuse of state resources and "overt media bias" in favour of Mr Yanukovych
State workers pressured to give absentee voting certificate to their superiors
Intimidation at some polling stations
Suspiciously high turnout - 96% - in the key pro-government region of Donetsk
About 10,000 protesters have also now gathered in front of Kiev's main railway station.
Government supporters, including several thousand miners, have held rival but smaller rallies.
"Calm your passions," Mr Kuchma appealed to the gathering crowds in a televised address.
"Any revolution must end with peace. The sooner this revolution, this so-called revolution, is over, the better it will be for the people whose fate concerns us so much."
Ukraine's Supreme Court will examine Mr Yushchenko's appeal against the electoral authority's decision on Monday, and insists no final assessment of the election can be made until then.
Serhiy Tyhipko, an aide to Mr Yanukovych, said he saw "no justification" for the result to be suspended.
Correspondents say the Supreme Court has in the past shown itself to be independent-minded.
It cannot invalidate the whole election, but it can force regional recounts.
Both sides of the dispute have lodged allegations of irregularities with the court.
Mr Solana's spokeswoman, Cristina Gallach, said he wanted to discuss "a negotiated diplomatic solution" to the crisis.
Mr Kwasniewski has drawn up a three-point plan to resolve the crisis: verifying the election results, annulling those tainted by irregularities, and the renunciation of violence by all sides.
Lech Walesa, the former Polish president who spearheaded the country's anti-communist movement in the 1980s, has said he believes a compromise is possible.
Before World War II, a large part of western Ukraine belonged to Poland and people there are keenly watching events unfold in Kiev.
But Russian Foreign Minister Sergei Lavrov has criticised European intervention in the crisis. He said a solution to the crisis could only be found "by the Ukrainian people themselves".
Correspondents point out that Moscow has made no secret of its support for Mr Yanukovych.
TV stations rebel
On Thursday, state-owned TV station UT1 rebelled against government control by announcing live on air that its news team was joining opposition protests.
Hours earlier, the private, formerly pro-government channel One Plus One said it would stop bending to government pressure in news reports.
Two TV stations have pledged to provide balanced coverage
Mr Yushchenko started the presidential campaign as Ukraine's most popular politician, and it took an enormous propaganda effort on state-run TV channels to make Mr Yanukovych look like a real contender, correspondents say.
The move may mean that pictures making plain the huge size of the opposition demonstrations will now reach the heartlands of Mr Yanukovych's support in the industrial east of the country, reports BBC world media correspondent Sebastian Usher.
So far, the country's coal miners there have refused to join a general strike, called by the opposition but reported to have had only limited impact so far.