Ukraine protesters in tense stand-off in central Kiev
Ukrainian riot police are taking up positions near Kiev's City Hall, where thousands of anti-government protesters have massed.
More police are being bussed in, the BBC's Steve Rosenberg says, with priests urging them not to use force.
The tense stand-off follows weeks of demonstrations after a government U-turn on a free trade deal with the EU.
President Viktor Yanukovych said a he would hold talks involving the opposition on Tuesday.
As another big crowd calls for closer relations with Europe, no resolution to the stand-off between the pro-EU protesters and the Ukrainian government is in sight.
The opposition is sticking to its demand for the replacement of what amounts to the entire ruling structure: the president, parliament and cabinet.
But President Viktor Yanukovych is not moving an inch either, forging ahead with plans for a "strategic partnership with Russia".
The government has state power on its side, but the protesters have a no less potent weapon: people power.
Tensions continue to mount, with two events on Sunday only adding to the strained atmosphere: an announcement by state security service officials that they are looking into an alleged attempt to seize power, and the toppling of Kiev's Lenin statue.
Catherine Ashton, the EU foreign policy head, will have her work cut out when she arrives in Kiev to try to defuse the situation.
A statement on the president's website said Mr Yanukovych would hold talks with three of his predecessors on Tuesday to try to find a compromise.
The protesters have given Mr Yanukovych 48 hours to dismiss the government and are demanding new elections for the presidency and government.
They have condemned Mr Yanukovych for refusing to sign an association agreement with the EU last month. He said he shelved it because it would put trade with Russia at risk.'Show restraint'
Protesters are blockading government buildings with cars, barricades and tents.
EU foreign policy chief Catherine Ashton will be in Ukraine on Tuesday and Wednesday "to support a way out of the political crisis".
President of the European Commission, Jose Manuel Barroso, said in a statement that he had spoken to Mr Yanukovych over the telephone and asked him "to show restraint".
He said he had asked him "to not use force against the people that are demonstrating peacefully, to respect fully the freedoms that are so important for all of us in Europe".
Baroness Ashton will hold talks with government officials, opposition activists and civil society groups.
The Commission says the EU's offer of an association agreement with Ukraine remains on the table, provided Ukraine meets the conditions - and they cannot be renegotiated.
One key condition is the release of opposition leader Yulia Tymoshenko, jailed in 2011 over a controversial gas contract with Russia.
Lenin and Ukraine
Ukraine's relationship with Vladimir Lenin is complicated.
He is still widely seen as a symbol of the Soviet occupation. He was the one who brought the nation back into the Kremlin's fold after its short-lived independence following the collapse of the Russian Empire in 1917.
A bloody Red Army operation resulted in the capture of two-thirds of Ukraine. A campaign of forced food requisitions - part of Lenin's "crusade for bread" - starved thousands of Ukrainians.
But Lenin also helped make Ukraine the industrial powerhouse it is today. His government approved Ukraine's first major hydropower plant on the Dnieper river at Zaporizhya. That, in turn, spawned a number of huge steel-making and chemical factories.
Hundreds of thousands of people took part in Sunday's protest - the biggest so far in nearly three weeks of opposition demonstrations in Kiev.
During the evening, a group of protesters smashed the city's statue of the Russian revolutionary leader Lenin, and brought its dismembered parts as trophies to Independence Square.
The statue stood at the top of Shevchenko Boulevard, and was toppled with metal bars and ropes, then smashed up with hammers. No arrests were reported in the incident.
These are the biggest street protests in Ukraine since the 2004 Orange Revolution, which swept pro-Western leaders to power, though Mr Yanukovych made a comeback in 2010.
Many of the protesters suspect Russia's President Vladimir Putin of trying to model a new Russian-led customs union on the Soviet Union. So far only Belarus and Kazakhstan have joined it.
The party of Ms Tymoshenko, a former prime minister, has urged people to "chase" the president "until he falls".
Meanwhile the Ukrainian Security Service said it was investigating some politicians on suspicion of what it called "actions aimed at seizing state power". It did not name the politicians.
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