Tens of thousands of Mauritians protested Saturday in the capital Saint-Louis over the government's handling of a giant oil spill off its pristine Indian Ocean coast.

The Japanese bulk carrier MV Wakashio crashed into a reef off southeastern Mauritius last month spewing more than 1,000 tonnes of oil into waters that are home to mangrove forests and endangered species. 

After the boat split in two, the larger piece was towed out to sea and sunk, but the smaller section remains stranded on the reef.

The call for the march came from an ordinary citizen, Jean Bruneau Laurette, who has become a hero among many for daring to oppose Prime Minister Pravind Jugnauth.

Laurette, a maritime security expert, says the government has been hiding the truth about the circumstances of the oil spill. He has filed a case against the environment ministry.

Up to 75,000 protesters thronged the square in front of the cathedral in downtown Port-Louis, an AFP reporter said, in the biggest demonstration in 40 years.

Many of them were dressed in black -- the colour of mourning. Public anger has boiled over in Mauritius after at least 34 melon-headed whales were found dead or seriously ill near the site of the spill.

Fisheries minister Sudheer Maudhoo had said there was "no trace of hydrocarbons on them or in their respiratory system".

"This rally is an occasion to send a message to tell Pravind Jugnauth he has messed up," marcher Jocelyne Leung, 35, told AFP.

"This is the first time that a citizens' demonstration has gathered such a big crowd," said Ajay Gunness, the number two of the opposition MMM party.

Many protesters carried the national flag, sang the national anthem, and called for Jugnauth to step down.

Authorities and experts from Japan and Britain are still investigating the true extent of the ecological damage to an island whose economy depends heavily on tourism.

This archipelago is a tourist haven and many of it's 1.3 population derive their livelihood from tourism or fishing.

Veteran politician Jugnauth, whose current stint in power began in 2017, has denied making any mistakes in handling the spill.