Cordons of riot police moved into central Kiev early on Monday afternoon in what appeared to be preparations by the Ukrainian government to regain control of Independence square and Kiev city hall, occupied by anti-government protesters for the past week.

The move comes after a week of protests against President Viktor Yanukovych's decision not to sign an integration pact with the EU, culminating in the biggest protest since the 2004 Orange Revolution on Sunday, when hundreds of thousands of people flooded central Kiev. The protest ended with the city's statue of Lenin toppled by protesters and attacked with hammers.

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

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

As the riot police moved into the centre of town on Monday, a priest conducted a religious service from the 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.

"We are here to keep order, we are not going to apply force," one of the officers near Independence square said, refusing to give his name. However, the Kyiv Post newspaper reported a "top government source" saying a decision had been taken to storm the square. Heavyweight boxer Vitali Klitschko, one of the three main opposition leaders, called on women and children to leave the square as rumours spread that it could be stormed.

A protester swings a hammer at the fallen statue of Lenin in central Kiev. Photograph: Sergei Chuzavkov/AP

Three central metro stations were closed over apparent bomb threats, with rumours spreading among the protesters that the metro could be used to deliver soldiers into central Kiev.

The protesters have dug in at the square, with a hardcore 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, protest leaders gave Yanukovych 48 hours to fire his prime minister, and said if he did not they would march on his country residence. There have also been demands for snap parliamentary and presidential elections.

As the riot police rumours 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.