Colombo: At least 25 people were killed and more than 100 others injured after violent clashes erupted inside a prison on the outskirts of Sri Lanka's capital, Colombo, officials and local media reported on Monday.

The unrest broke out at the prison in Negombo, around 35 kilometres north of Colombo, on Sunday and continued into Monday.

Police spokesperson Chandana Herath confirmed that there had been fatalities but did not immediately verify the death toll reported by local television channel Hiru.

According to the broadcaster, those killed included both inmates and prison officials.

The cause of the violence was not immediately clear, and authorities have not yet disclosed what triggered the clashes.

An official at the main state-run hospital in the area said more than 100 people had been admitted with injuries sustained during the prison violence. Speaking on condition of anonymity, the official declined to provide further details about the nature of the injuries, saying they were not authorised to speak to the media.

Authorities are continuing to monitor the situation as investigations into the incident get under way.

Sri Lanka's prison system has long struggled with severe overcrowding. More than 39,000 inmates are currently housed in facilities designed to accommodate only around 10,000 prisoners, raising persistent concerns over safety and prison conditions.