Nouakchott - Mauritanians began voting on Saturday on several contentious changes to their constitution sought by President Mohamed Ould Abdel Aziz but opposed by a wide swathe of opposition lawmakers and civil society groups.
Aziz is pushing to abolish the country's Senate and several other state bodies and to make a small alteration to the national flag, measures that have galvanised a boycott movement hoping to sap the vote's credibility by forcing a low turnout.
Polling stations opened at 07:00 and were due to close at 19:00 in the conservative west African nation, where violent clashes have broken out after the authorities banned several rallies planned by opponents of the changes.
The president is due to vote between 08:00 and 09:00, but few others were visible casting their ballots in several Nouakchott polling stations visited by an AFP journalist at opening.
While Aziz, his supporters and several opposition parties are seeking a "Yes" vote, one moderate opposition party is seeking a "No", while the remaining parties have joined civil society groups to call for a total boycott for amendments they consider unconstitutional.
Jemil Ould Mansour, head of the Islamic Tewassoul party spearheading the movement against the vote, said the country's leaders had fixed "the electoral roll and voting materials to prepare fraud on a massive scale," and warned of violence.
"Those in power are pushing people to violence by not allowing any kind of protest not in line with their own views," Mansour told reporters on Friday.
The boycott movement draws broad political support from figures as diverse as religious conservatives and anti-slavery activists.
They have held several protests attracting thousands of supporters, but have also been prevented from demonstrating by the security forces, who on Thursday shut down several planned rallies close to the capital with tear gas and baton rounds.
The UN Human Rights Office said Thursday that "protest leaders were reportedly beaten up and a number of them were arrested" during campaign rallies in the last few weeks, urging the government "take all necessary measures to ensure free, transparent and credible elections".
Around 1.4 million Mauritanians are eligible to vote, and results are expected early next week.
The proposal to modify the constitution, in force since 1991, was approved by lawmakers in the lower house but rejected by 33 out of 56 senators in March, leading Aziz to call the referendum to push through the changes.
The most contentious measure remains the abolition of the Senate and its replacement with elected regional councils, and Senators have held a sit-in this week and demanded apologies from Aziz for accusing them of corruption.
Unrepentant, Aziz said at a final rally on Thursday the Senate "costs a lot, has no use and does nothing", and vowed to launch legal action against several senators.
The opposition also fears that despite Aziz's claims to the contrary he is laying the groundwork for a third term in power, as his own prime minister said back in July he supported the idea.
The president came to power by coup in 2008 and was elected in 2009 and again in 2014 for a second five-year term.
There will be two ballot boxes on Saturday, one for the institutional changes and the other for an alteration to the national flag.
The current green flag with yellow Islamic crescent and star would be altered to honour those who fought for freedom from colonial master France by a red band at the top and bottom, representing blood spilt for their nation, if the measure is passed.
Mauritania won independence in 1960.