At least 15 prisoners and 4 guards were killed in a riot Sunday evening at a prison on the outskirts of the Sri Lankan capital Colombo, a medical source said Monday.

AFP quoted Negombo Hospital director Pushpa Gamalath as saying that more than 100 other prisoners were taken to hospital following the riot. Gamalath added that 'some victims were shot.'

Sri Lankan authorities reported that the events began Sunday evening when two groups of prisoners, described as rival drug trafficking gangs, clashed inside the facility, which houses about 10,000 inmates.

A police officer, speaking to AFP on condition of anonymity, said the situation was completely out of control and that four guards were killed while trying to contain the riot.

Special police units were called in as reinforcements but did not enter the prison grounds, while many relatives of inmates gathered outside the facility.

In December 2020, a prison riot in Sri Lanka during the COVID-19 pandemic killed 11 people and injured 117, prompting the government to release hundreds of prisoners.

According to official statistics, the number of prisoners in Sri Lanka stands at 41,250, four times the capacity of the prisons.