The spreading disorder set off a new round of speculation that Mr. Yanukovich would declare a state of emergency and potentially turn again to force by ordering the removal of demonstrators who have occupied Independence Square and several public buildings, including Kiev’s City Hall. There were reports on Sunday that the security services were preparing to bring charges of treason against three opposition leaders in Parliament who have been at the forefront of the demonstrations.

One of those leaders, Arseniy P. Yatsenyuk, of the opposition Fatherland party, called for mass civil disobedience if Mr. Yanukovich tried to impose martial law. “In the case that a state of emergency is declared, everyone should go to Maidan,” Mr. Yatsenyuk said, referring to Independence Square.

Mr. Yanukovich only added to the demonstrators’ anger by stopping on his way back from China on Friday to meet with President Vladimir V. Putin of Russia in Sochi, the Russian resort city. Rumors immediately began swirling that Mr. Yanukovich had cut a secret economic deal with Mr. Putin that would lead to Ukraine joining a customs union with Russia, Belarus and Kazakhstan, a step that many Ukrainians believe would deliver their country’s future into the Kremlin’s hands. Both governments denied the rumors, but the protesters do not trust either Mr. Putin or Mr. Yanukovich, and opposition leaders used the matter on Sunday to whip up the crowd in Kiev.

“Today, they fall on their knees in front of the president of Russia and surrender us to the customs union,” said Oleg Tyagnibok, the leader of the nationalist Svoboda Party. “We demand to make public what these secret negotiations were about. They bring us back to the time of Stalinism. Is this 1937?”

Later, as the Lenin statue was pulled down and men took turns splintering it to bits with a sledgehammer, protesters twice sang the national anthem, removing their caps and covering their hearts with their hands. One of the hammerers wore his hair in a mohawk; another was a priest in black vestments. Onlookers shielded their faces from the flying granite chips as they cheered them on, yelling: “Good job, guys.”