The US has censured the use of force to disperse crowds in Azerbaijan's capital Baku on Saturday who were protesting about recent parliamentary elections.
Hundreds of people are said to have been injured in the crackdown
A statement by the US embassy in Baku said it deplored the "unjustified and unprovoked use of force".
Many people were reported to have been injured by police using truncheons, tear gas and water cannon.
The opposition say the results giving President Ilham Aliyev's party overwhelming victory were falsified.
Leaders of the opposition United Freedom Bloc have insisted on a re-run of the vote.
Saturday saw the first use of police force against a rally since the 6 November election.
Hundreds of policemen in full riot gear had lined the edges of Victory Square for the latest of a series of tightly controlled demonstrations.
"We deplore the unjustified and unprovoked use of force against citizens peacefully exercising their right to freedom of assembly," said the statement issued by the US embassy in Baku.
Police chiefs said only one demonstrator sustained injuries, while 20 officers were seriously hurt.
They branded the demonstrators, who defied a two-hour limit on their rally, as "provocateurs" and denied using water cannons or tear gas.
"They were calling for disobedience... we warned them," said Baku deputy police chief Yashar Aliyev.
He said 29 people were arrested, including a leader of the opposition Liberal party.
Many activists, inspired by Ukraine's "Orange Revolution" of a year ago and believing the time has come to put more pressure on the government, had called for an open-ended protest.
But President Aliyev, son of the late Heydar Aliyev, says that in his oil-rich state the chances of such a revolution are zero.