At the height of the unrest Sunday night, a seething crowd toppled and smashed a statue of Lenin, the most prominent monument to the Communist leader in Kiev. The act was heavy with symbolism, underscoring the protesters’ rage at Russia over its role in the events that first prompted the protests: Mr. Yanukovich’s abrupt refusal to sign sweeping political and free-trade agreements with the European Union.

After an electrifying assembly in Independence Square in the center of Kiev, the main focus of the protests, the huge crowd surged across the capital, erecting barriers to block the streets around the presidential headquarters and pitching huge tents in strategic intersections. They were not challenged by the police, who have largely disengaged since their bloody crackdown on a group of protesters on Nov. 30 sharply increased outrage at the government.

International concern over the unrest in Ukraine appeared to deepen on Sunday, as the United Nations secretary general, Ban Ki-moon, telephoned Mr. Yanukovich and Western leaders continued to call on him to respond to the demonstrators’ demands. The European Union has been eager to draw Ukraine, a nation of 46 million, into closer alliance with the West, while Russia has sought to safeguard its major economic and political interests in its close neighbor. Making the crisis more acute, Ukraine is teetering on the verge of bankruptcy and is desperate for financial assistance from abroad.