Nouakchott - Police and opposition activists clashed on Thursday in Mauritania's capital on the final day of campaigning for a controversial constitutional referendum.

The authorities refused to give the green light for protests opposing the constitutional changes in Nouakchott and three regions of the west African country, leading to what activists said was "excessive" use of tear gas in an area to the west of the capital.

A coalition drawn from a broad political spectrum including Islamists and anti-slavery activists in the conservative west African nation opposes measures including abolishing the senate and changing the national flag, and some are boycotting Saturday's vote.

Saleh Ould Henenna, a spokesperson for the protesters, said "we have to inform the authorities of our activities but we don't have to wait for their approval," denouncing "the dictatorship which governs this country".

Activists including the head of an anti-slavery group had fainted during the rallies because of the tear gas, he said.

The clashes came hours before President Mohamed Ould Abdel Aziz used a rally in the capital to denounce senators opposed to constitutional changes.

He accused them of "betraying the nation" as well as levelling other charges against them.

The 60-year-old called on the crowd to demand the dissolution of the senate which, he said, "costs a lot, has no use and does nothing".

He also vowed to launch legal action relating to "far more serious cases", without giving further details.

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.

But the opposition fears that despite Aziz's claims to the contrary he is laying the groundwork for a third term in power, with his own prime minister saying recently he supported the idea.

Around 20 senators maintained a sit-in at the chamber on Thursday, under police guard.

Opposition lawmakers from the lower house have followed suit in solidarity with their peers, a parliamentary source told AFP.