Thousands of people marched on Saturday in Port Louis, the capital city of Mauritius, a month after a grounded Japanese ship leaked over 1,000 tons of fuel oil into the island’s fragile marine areas.
Many wore black and waved the national flag, while honking horns and drumming.
The protesters called for the government to resign and had T-shirts with the inscription: “I love my country. I’m ashamed of my government.”
It is the biggest protest the country has seen in recent years.
Many Mauritians believe the government could have done more to prevent the spill.
There is also criticism over the decision to deliberately sink part of the ship after it split in two.
One woman told the BBC’s Yasine Mohabuth: “I am present today because we want the truth.
“They didn’t do anything when the ship approached our coastline – 12 days they didn’t do anything until the oil spill and now thousands of people and marine people are affected.”
Mauritians in the diaspora also held demonstrations in cities including London, Paris and Perth.
The government has promised to set up a commission to investigate the spill.
The captain of the ship has been arrested and charged with endangering safe navigation. He has not yet commented.
At least 39 dead dolphins have washed ashore and experts fear the fuel’s chemicals may be to blame.
There are concerns that the wreckage could also weaken coral reefs and the fragile ecosystem of the island’s mangroves.