Police in Ukraine began moving against protesters in central Kiev on Monday, with some protest camps in front of government buildings being dismantled.
No clashes have been reported, but opposition leaders are urging supporters to defend Independence Square, the main protest site.
Officials have given the protesters until Tuesday to leave.
A BBC correspondent in Kiev said there were far more police in the city centre than at the weekend, when hundreds of thousands of people came out onto the streets.
The stand-off follows weeks of unrest after a government U-turn on a free-trade deal with the European Union.
Prosecutor-General Viktor Pshonka warned protesters to stop causing "anarchy and lawlessness" by blocking the buildings.
President Viktor Yanukovych has said he will discuss the crisis with three former presidents 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.