Mauritanians have turned out in large numbers to vote in a referendum which could herald a transformation of the country's political landscape.
Mauritanian voters have not had much of a say so far
Mauritania - a desert state that straddles Arab and black Africa - has never had a peaceful change of power.
But the current military leader has promised to leave power and hold free elections next year.
The country's foreign ministry said it estimated turnout at up to 70% by the time polls closed, Reuters reported.
Preliminary results were expected overnight, and observers suggested the reforms would win a comfortable majority of votes.
The junta which seized power in Mauritania in a bloodless military coup last August overthrew an unpopular government which had been in power for 20 years.
The new strongman, Colonel Elly Ould Mohamed Vall, immediately began liberal reforms, including the promise of free elections and the formation of a national committee to monitor the distribution of revenue from offshore oilfields which came on stream early this year.
"It is a historic day for Mauritania. It is truly the rebirth of politics in this country," he said after casting his vote in the capital, Nouakchott.
At first the international community condemned the latest military coup but it is now watching with interest to see if the reforms are followed through.
Col Vall - a former intelligence chief - says he will not stand in the presidential elections, which are due to be held next March assuming the constitution is adopted in the referendum.
Mauritania is a chronically unstable, extremely poor country with Arab tribes that often show more loyalty to their chiefs than to the state.
Black African Mauritanians say they are oppressed by marginalisation and even slavery.
Presidents limited to two consecutive terms
Terms cut from six years to five
Maximum presidential age limit of 75 imposed
One of the African political parties has called for a boycott of the referendum saying it does not address inequalities in the country.
Although Colonel Vall's reforms have marked him out as very different from previous military leaders in Mauritania, veteran opponents of the government say they are disappointed he has not investigated allegations of corruption against former governments.
A few days before the vote, the authorities said they had arrested a group of people connected with the government overthrown last year. They were accused of plotting to destabilise the country.