The head of a boycott movement opposed to a constitutional referendum in Mauritania accused the country's rulers on Friday of planning "massive fraud" and warned of violence.
Saturday's referendum follows a tense campaign punctuated by massive protests that have been at times violently put down by the security forces.
Jemil Ould Mansour, head of the Islamic Tewassoul party spearheading the movement against the vote, said President Mohamed Ould Abdel Aziz and his supporters had fixed "the electoral roll and voting materials to prepare fraud on a massive scale."
"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.
The boycott draws broad political support from religious conservatives and anti-slavery activists in the conservative west African nation, all of whom oppose measures including abolishing the Senate, suppressing several state bodies, and changing the national flag.
Around 1.4 million Mauritanians are eligible to vote, but the boycott coalition has declared the changes unconstitutional and is urging citizens to stay at home.
Violence "is not our choice", Mansour told journalists, but added Aziz and his supporters would be responsible if they continued to "force" the changes through.
Police and opposition activists clashed Thursday in Mauritania's capital on the final day of campaigning after the authorities refused to give the green light for protests in the capital and other regions of the Islamic republic.
Senators rejected the abolition of their own chamber in March, apparently to the government's surprise as a majority are from the ruling party, prompting Aziz to call the referendum.
Aziz accused the lawmakers of "betraying the nation" at his final rally, and called on the crowd to demand the dissolution of a chamber which, he said, "costs a lot, has no use and does nothing".
Several senators would soon be on trial, the president said, accusing them of corruption.
Polling stations open at 7am on Saturday while the nation's armed forces cast their ballots on Friday, according the Independent National Electoral Commission (CENI).
Results are due early next week.