Ukrainian police fired tear gas Monday at pro-European demonstrators staging a second day of protests outside the government seat in Kiev over a decision to scrap a key pact with the EU.
Scuffles broke out outside the government headquarters shortly before the European Union said the offer of a broad political and economic deal this week was "still on the table".
A day earlier, tens of thousands swarmed central Kiev chanting 'revolution' and waving flags, in the biggest rally since the 2004 Orange Revolution overturned a rigged presidential poll and forced a new ballot.
Despite rain up to 1,000 protesters returned Monday to force President Viktor Yanukovych to sign the agreement at a summit in Vilnius, demanding the government's resignation.
Some attempted to enter the government building but were forced back by riot police with tear gas, AFP correspondents said.
The authorities later said the tear gas was used by opposition activists, not police.
Protest leaders said the attempted storming of the government building -- for the second time in two days -- was a "provocation" caused by unknown troublemakers.
A bigger protest is set for Monday evening.
"We will continue our protests until the agreement is signed," world boxing champion and opposition leader Vitali Klitschko told the crowd.
Ultra-nationalist party Svoboda said one of its activists had his arm broken by riot police.
Some 15,000 also rallied in the fiercely pro-Western city of Lviv, while some 12,000 protested in the nearby city of Ivano-Frankivsk.
In Kiev, the opposition have set up around 20 tents and vowed to remain there until Yanukovych signs the agreement.
"People took to the streets to defend Ukraine's European choice," protester Natalia Yagoda said.
In a bid to claim immunity from arrest, protesters put up signs on some of the tents turning them into makeshift lawmakers' offices.
Last week the government halted work to sign the Association Agreement, seen as a first step toward EU membership that would have marked a historic break from the Kremlin.
The decision came after the parliament failed to adopt legislation that would have freed jailed opposition leader Yulia Tymoshenko, a key EU condition for the signing of the agreement.
The government cited "national security" for its decision, but faced accusations from the West that it had caved into pressure from Moscow.
The EU Monday reiterated its criticism of Russian pressure on Ukraine over the accord.
"It is up to Ukraine to freely decide what kind of engagement (it seeks) with the EU," EU President Herman Van Rompuy and European Commission head Jose Manuel Barroso said in a statement.
The EU "position remains clear. The offer of signing an unprecedented Association Agreement and (trade accord) is still on the table."
The Kremlin wants Ukraine to join its Customs Union which also includes Belarus and Kazakshtan and threatened with retaliation if Kiev signed the deal.
Linguistic and historic fault lines divide Ukraine into the Kremlin-friendly Russian-speaking east and the nationalistic Ukrainian-speaking west, and Yanukovych's decision to put the negotiations on ice is expected to once again polarise the nation.
According to a November poll by Gfk Ukraine, 45 percent of respondents said the country should sign the EU deal, while 14 percent supported Moscow's Customs Union membership.
Police put attendance at Sunday's main rally at around 50,000 people. The opposition said more than 100,000 turned out.
The jailed Tymoshenko urged Ukrainians to protest until the government signs the agreement.
After losing to Yanukovych in a 2010 poll, the former prime minister was sentenced to seven years in jail on abuse of power charges seen by the West as politically motivated.
The government explained its decision to drop the EU agreement as the result of economic hardship.
In an interview with Russian television, Prime Minister Mykola Azarov said the EU had denied Ukraine proper financial support for integration, promising just one billion euros over seven years.
But many observers say Yanukovych's about face was motivated by fears for his personal future and that he has no interest in freeing Tymoshenko ahead of 2015 presidential elections.
The government's sudden turn away from the EU deal came after Yanukovych this month travelled to Moscow for secret talks with Putin.
"For Yanukovych and all of his eastern neighbours there's nothing more important that staying in power," Russian daily Vedomosti said Monday.
"Yanukovych does not care what and which union Ukraine will be part of if he is not at the helm of it."