Sri Lanka's Prime Minister Mahinda Rajapaksa has resigned amid mass protests at the government's handling of the economic crisis, officials say.

The move came as the island was placed under curfew after violent clashes between Rajapksa supporters and anti-government protesters in Colombo.

At least 78 people have been injured in the violence in the capital, a local hospital says.

There have been protests over soaring prices and power cuts since last month.

Mr Rajapaksa, 76, sent his resignation letter to his younger brother President Gotabaya Rajapaksa, his spokesman said.

The island nation is facing its worst economic crisis since gaining independence from Britain in 1948.

The government has requested emergency financial help. Many Sri Lankans also want President Rajapaksa to go as well.