The U.S. is concerned by the detention of human rights activists, who gathered for an unsanctioned rally in downtown Moscow, a spokesman for the National Security Council said on Friday.

Police arrested over 50 people for an attempt to hold an unauthorized protest meeting near Moscow's Triumph Square late on Thursday.

"The United States expresses dismay at reports that authorities in Moscow prevented Russian citizens from exercising their right to assemble peacefully," Mike Hammer said in a statement.

A leader of the opposition movement The Other Russia, Eduard Limonov, and 82-year-old head of the Moscow Helsinki group, Lyudmila Alexeyeva, are said to be among those detained.

"In particular, the United States notes with concern the detention of protesters, including prominent human rights defender Ludmila Alexeeva, and reports of their mistreatment by authorities while in custody. Freedom of speech and assembly are universal rights that all governments should recognize and defend," the statement says.

Moscow police have been on high alert due to mass celebrations on the New Year's Eve as some 30,000 people have gathered in downtown Moscow for traditional New Year's festivities, which included a concert on Red Square. Some 4,000 additional law enforcement officers were on patrol in downtown Moscow.

WASHINGTON, January 1 (RIA Novosti)