Tens of thousands of Russians took to the streets in Moscow Saturday to protest national election results that appeared to keep the ruling party in power. The New York Times said the demonstrations forced the government to confront a level of discontent unseen in the country since Vladimir Putin, who has been prime minister since May 2008, became acting president in December 1999 after Boris Yeltsin resigned unexpectedly. The crowd was so large it overflowed from a central city square, forcing people on the edges to climb trees or watch from an opposite riverbank, the Times said. Protest leaders promised to gather an even larger crowd Dec. 24. The evening news on television does not ordinarily report criticism of Putin, the Times said, but the protest was too large to be blacked out, and was accompanied by dozens of smaller rallies across Russia\'s nine time zones. Moscow police put the crowd size at 25,000, but protest organizers said there were more than twice that many. About 50,000 police and riot troops were in place well before crowds began gathering, the BBC reported. Demonstrations also took place in the country\'s second-largest city, St. Petersburg, the southern Siberian city of Barnaul, Krasnoyarsk, Vladivostok, Chita and Khabarovsk, RIA Novosti said. Simmering suspicions of voting irregularities were heightened when U.S. Secretary of State Hillary Clinton said the U.S. government had concerns about the election. Russia\'s presidential Council for Human Rights also expressed concerns Friday to President Dmitry Medvedev, although it has no power to order another vote, the BBC said. Since the election, the Communist Party posted a notice on its Web site it does not recognize the results of the election, as did the liberal Yabloko party, which said it would take legal action to dispute the results, RIA Novosti said.