Mauritanian police have fired tear gas at protesters attempting to hold a "day of rage" against President Mohamed Ould Abdelaziz's government.
A BBC reporter in the capital says police prevented hundreds of people from entering one of the main squares.
Twenty two people were arrested and opposition MPs were prevented from joining the demonstrators.
Some Mauritanians have been holding regular protests against authorities in Nouakchott since February.
Messages uploaded to the video-sharing website, YouTube, were posted to encourage Mauritanians to turn out.
The BBC's Hamdi Mohammed in Nouakchott says the demonstration stretched for nearly a mile down the city's main street before police moved in.
There was a mixture of men and women waving national flags and chanting slogans calling for reforms, saying that President Abdelaziz holds too much power.
Some people showed support for the protesters in midday heat from the sidelines by handing out bottles of water, our reporter says.
A "day of rage" protest was also held in the northern city of Nouadhibou.
Gen Abdelaziz came to power by ousting his democratically elected predecessor in a military coup in August 2008.
Nearly a year later he won his own democratic mandate in elections held under an agreement with coup opponents.