The double-decker vessel, which was carrying more than 30 passengers, overturned off a beach near the coastal town of Tanur in the Malappuram district of Kerala state.

Police think it was overcrowded when it got into trouble at around 7pm local time on Sunday.

A criminal case has been launched against the boat's owner, Nazar Pattarakath from Tanur, according to local media.

About 10 passengers were taken to hospital for treatment, with four of those in critical condition.

Officials said they feared that some people might be still missing.

Most of the passengers were tourists.

The dead included children who had come for a ride on the boat during their school holidays, Kerala's sports minister V Abdurahiman said.

The minister told reporters that the boat was stuck in the muddy waters and attempts were being made to pull it out.

"The boat is being hauled ashore and more bodies are expected to be recovered from inside," he told the Press Trust of India news agency.

A team from the National Disaster Response Force was at the site, a Kerala State Disaster Management Authority spokesperson said. The Indian Coast Guard was also involved in the rescue mission.

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Monday has been declared a day of official mourning, according to local media reports.

Prime Minister Narendra Modi tweeted: "Pained by the loss of lives due to the boat mishap in Malappuram, Kerala."

He also announced compensation for the families of the victims.

Boating accidents are common in India, where many vessels are overcrowded and have inadequate safety equipment.

In September 2020, 12 people drowned when a sightseeing boat capsized on the swollen Godavari River in the southern state of Andhra Pradesh.

In May 2018, 30 people died when a boat capsized in the same region.