The authorities reported Sunday night that about a hundred police officers and more than 50 protesters were injured, including some with burns to their eyes from tear gas. Witnesses said that some protesters, including women, were beaten brutally.

The police made scattered arrests, but did not immediately release a tally.

Protest leaders said they intended to make Mr. Yanukovich a prisoner in his own capital on Monday, with streets blocked, government buildings surrounded or occupied and possibly a general strike by workers and students. A map was posted on Facebook showing supporters where to put their vehicles to obstruct traffic.

By Sunday evening, demonstrators had taken police barriers meant to keep them out of Independence Square and repositioned them to mark their own control of a large area in the city center.

There were demonstrations in cities like Lviv and Chernivtsi, in the generally pro-European western part of Ukraine, and also in the predominantly Russian-speaking east, which tends to favor close ties with Moscow and where Mr. Yanukovich has his main base of support. Several thousand people rallied in Dnipropetrovsk in the southeast, defying a court order banning a protest there. Even in Donetsk, Mr. Yanukovich’s hometown in the east, hundreds rallied in favor of European integration.

Serhiy Lyovochkin, the chief of the presidential administration staff, reportedly resigned on Saturday over the crackdown. At least five lawmakers from the Party of Regions spoke out forcefully against the police violence, and at least two, David Zhvania and Inna Bohoslovska, said they had quit the party. Ms. Bohoslovska sent a text message to a protest leader, Yegor Sobolev, telling him: “If I can be useful, I am here. Let’s go to the rally.”

There were signs that some of Ukraine’s wealthiest business leaders, known as oligarchs, were turning against Mr. Yanukovich as well, or at least were positioning themselves for a major shift in the government. Mr. Sobolev said that a television channel owned by Rinat Akhmetov, Ukraine’s richest man, invited him to appear in prime time, which he took to be an effort by Mr. Akhmetov to reach out to the opposition.

The Orange Revolution in 2004 also centered on mass protests against Mr. Yanukovich, in response to blatant election fraud that made him the easy winner of the presidential election that year, contradicting early returns and exit polls that showed him losing to Viktor A. Yushchenko. The protests led to a new election that Mr. Yushchenko won, but Mr. Yanukovich made a comeback in 2010, defeating Yulia V. Tymoshenko, the former prime minister. Ms. Tymoshenko has since been prosecuted and imprisoned for abuse of authority.