Police and protesters opposed to next week's presidential election clashed in the capital Nouakchott on Thursday, leaving several people hurt and others arrested.
Officers used tear gas and batons to break up the demonstration, and protesters responded by throwing stones at and fighting with the officers.
As the clashes continued, the demonstration broke up into several groups of hundreds of people, mainly youths, who attacked police.
The fighting lasted more than an hour and hospital officials told AFP that several people were admitted with injuries, without saying if they were police or demonstrators, or giving numbers.
"We don't yet have a precise number but we know that a lot of our supporters are in hospital," Jemil Ould Mansour, the leader of the Islamist Tawassol party, told reporters.
Opposition leader Ahmed Ould Daddah and the speaker of parliament Messaoud Ould Boulkheir had led off the march at 6:00 pm (1800 GMT) in protest against the election being organised by the military regime.
The speaker has refused to take up his position in the National Assembly since the army took power, on August 6, 2008.
Leaders of Mauritania's anti-coup parties have denounced the June 6 election as a charade designed to install the coup leader General Mohamed Ould Abdel Aziz in power and are boycotting the election.
International mediators have been trying to get the elections postponed so that they can be convinced to come on board.
Thursday's clashes came the same day United Nations, European Union, African Union, and Arab League officials met Mauritanian politicians in the Senegalese capital to try to break the deadlock over the election.
In a press conference after Thursday evening's clashes, Boulkheir called on his supporters to take to the streets in greater numbers.
"The blood of the demonstrators, their tears and their sweat will not have been shed in vain, but put at the service of the people struggling against arbitrary power," he said.
Before leaving for Dakar, opposition leaders had called on their supporters to step up their demonstrators in the streets.