Ukrainian authorities closed in on protesters in Kiev and raided the offices of a leading opposition party on Monday, in their first tentative attempt to exert control over the city centre that has been occupied by anti-government demonstrators for a week.

Riot police removed barricades around a key government building and stood face to face with lines of protesters as temperatures dropped and a blizzard swirled through Kiev. Despite widespread rumours, there was no attempt to regain control of Independence Square or the city hall, but police edged their lines closer during the day.

Activists from Fatherland, the political party of the jailed former prime minister Yulia Tymoshenko, said their offices had been stormed by armed men. Ostap Semerak, an MP, told the Guardian that heavily armed operatives from the SBU secret services arrived at the party headquarters and entered through the windows "like pirates or terrorists", before taking away computer servers.

He said: "This is a building where several MPs have their offices and the act was completely illegal. If there was a court order then they should have shown it, but this kind of behaviour is outrageous."

A week of protests against President Viktor Yanukovych's decision not to sign an integration pact with the EU culminated on Sunday, when hundreds of thousands of people flooded central Kiev in the biggest demonstration since the 2004 Orange Revolution. Protesters toppled a statue of Lenin and attacked it with hammers.

Yanukovych released a statement on Monday saying he supported the idea of an "all-nation roundtable", including three former presidents, to find a solution to the crisis. "Such a roundtable could lead to more understanding," said a statement from the presidential press service.

The EU's top diplomat, Lady Ashton, will arrive in Kiev on Tuesday. The European commission president José Manuel Barroso said Ashton would try to help defuse "the very tense situation that Ukraine is living today".

Barroso also voiced support for the protesters, writing on Twitter: "Those young people in the streets of Ukraine, with freezing temperatures, are writing the new narrative for Europe."

Sweden's foreign minister, Carl Bildt, said Ashton should seek to "strengthen reforms, democracy and economy in Ukraine". He wrote on Twitter that the storming of the Fatherland offices was "utterly unacceptable in European society".

As the riot police moved into the centre of the city on Monday lunchtime, a priest conducted a religious service from a stage, accompanied by chanting from the assembled crowd – a fraction of Sunday's masses but still numbering several thousand.

"It's the third time I'm here and I'm not afraid," said Bogdan Tsap, 66, from Stary Sambir in western Ukraine, standing by his tent. Several lines of riot police and interior ministry troops wearing helmets and holding shields blocked the roads leading to Independence Square and city hall, and trucks mounted with water cannons were parked nearby.

The Kyiv Post newspaper reported a "top government source" as saying a decision had been taken to storm the square, but by midnight this had not happened.

In the early hours of the morning a few dozen policemen even began chatting to protesters and drinking tea on the edges of Independence Square, though further in barricades were topped with barbed wire and guarded by men wearing balaclavas and holding wooden plans.

It was unclear whether government officials had changed their minds or whether the police moves were an attempt to show the government could exert a modicum of control over the capital after the chaos of recent days.

The prime minister, Mykola Azarov, who survived a no-confidence vote in parliament last week, on Monday compared the toppling of the Lenin statue to the Taliban's destruction of Buddha monuments in Bamiyan, Afghanistan.

The protesters have dug in at Independence Square, with a core of several hundred setting up a makeshift camp with tents, log fires and soup kitchens while a large stage blasts pop music and speeches by opposition leaders. On Sunday the protest leaders gave Yanukovych 48 hours to sack Azarov and threatened to march on his country residence if he failed to do so. There have also been demands for snap parliamentary and presidential elections.

The protesters' demands have fluctuated. On Monday the heavyweight boxer and opposition leader Vitali Klitschko said the three key demands of the opposition now were punishing those responsible for attacking peaceful protesters on 30 November, releasing those arrested after scuffles with riot police, and the resignation of the government. "If the president applies force, he will be personally responsible," said Klitschko, reiterating that the protest was peaceful.

As rumours of a storm spread, civilians were evacuated from city hall, which has become a makeshift dormitory for protesters since it was occupied a week ago, though hundreds later returned. A few people in hard hats manned barricades on the stairs.

Vladimir Savchenko, 35, brandishing a table leg that he had weaponised with screws, said he was ready to defend the building if police stormed it. "We are not students, we are ready to fight back, I'm ready to die here if necessary," he said. "I don't know who I would vote for instead but I don't want this corrupt government."

Volunteer medics in white coats rushed around the building preparing masks for a potential teargas attack. As night fell the atmosphere across the city remained tense.

The far-right nationalist leader Oleh Tyahnibok claimed that operatives from the Russian FSB special services were in Ukraine preparing provocations, though did not provide any evidence.