Riots at a prison in western Sri Lanka have killed 26 people, seven of them guards, and injured more than 100.

The two days of violence at Negombo Prison, in a coastal town north of Colombo, began with clashes between two groups of inmates.

Prisoners are alleged to have grabbed guards' guns on Sunday. Two people were killed that day, with dozens injured. Later, groups of male prisoners and women from an adjoining unit climbed onto prison rooftops demanding to be released.

Fresh violence erupted on Monday when inmates tried to rush the prison gates. Security forces were deployed and multiple gunshots were heard from inside the prison.

The violence was reportedly triggered by a clash over an inmate informing on a drug trafficking operation inside.

A total of 23 prison officers and 54 inmates are still receiving treatment, officials said.

Some of the wounded were taken to Negombo Hospital. The hospital director told the AFP news agency that some had bullet wounds, while others had sustained cuts and severe bruises. Others were transported to Colombo National Hospital.

Police said part of the prison's roof collapsed during the incident, which injured some of the women involved, AFP reported.

Crowds of inmates' relatives gathered outside on Monday as the situation unfolded. Some told BBC Sinhala they had not been given information about where their relatives were, or who was alive and who had died.

Areas within the prison are still being cleared, and the military has been placed on standby after the police requested support, Reuters reported.

Prisons commissioner's media spokesman, AC Gajanayake, said steps had been taken to transfer inmates to other prisons.

Minister of Justice and Prisons Harshana Nanayakkara has instructed authorities to investigate and submit a report.

While riots occasionally break out in the country's overcrowded prisons, these were the worst in years. In December 2020, 11 were killed at another facility, with 117 injured.

Sri Lankan prisons held 41,250 inmates as of Sunday, which is four times their capacity, according to AFP.