A sea of colourful demonstrators waving flags and chanting slogans descended on Mahebourg on the southeast coast, where a cargo ship ran aground in July and leaked more than 1,000 tonnes of fuel into the pristine sea.
Police said 25,000 attended but organisers estimated the turn-out was at least double that.
The spill has inflicted untold damage on the Indian Ocean archipelago of 1.3 million people that depends on its fabled coastline for fishing and ecotourism.
Prime Minister Pravind Jugnauth and his government have been accused of not acting fast enough to prevent the worst environmental crisis in the country’s history.
Demonstrators thronging the coastline at Mahebourg, many with painted faces, chanted “He Must Leave!” and waved placards calling for the government to resign over the disaster.
“We are here to call on the government to pack their things and go. The people no longer trust this government,” said Marie, a protester who only gave her first name.
It was the second major rally over the spill after tens of thousands marched in the capital Port Louis on August 29 in the biggest demonstration witnessed in Mauritius in 40 years.
“We will keep up the pressure on the unpopular government of Pravind Jugnauth,” said Ashok Subron, one of the protest organisers
The MV Wakashio crashed off Mauritius on July 25 with 4,000 tonnes of fuel aboard but did not begin leaking oil for more than a week.
By the time Jugnauth issued an urgent appeal for international help the slick had reached the shore, coating mangrove forests, fragile ecosystems and coral reefs.
An army of volunteers scrubbed the coastline but the stricken bulker kept leaking, even after salvage crews declared the last of the fuel aboard having been removed on August 12.
“The MV Wakashio (incident) illustrates the incompetence of this government,” said Bruno Laurette, another protest organiser.
“Criminal negligence has had an impact on the flora and fauna of our country. Enough. They have to be put out of harm’s way.”
The ship eventually split and the larger piece was towed out to sea and sunk, further angering conservationists. The smaller section remains stranded on the reef and is visible from Mahebourg.
The washing ashore of nearly 50 dead melon-headed whales only spurred further outrage, as did the death of two sailors involved in the salvage process when their tugboat collided with a barge.
The Japanese owner of the MV Wakashio pledged this week to pay at least $9.4 million to help fix the damage caused by the spill.
It is still unclear why the Wakashio was so close to shore when the accident occurred. Jugnauth has commissioned a formal investigation and promised a full and transparent inquiry.