Sri Lanka's police announced Sunday the arrests of nearly 15,000 people during a week-long military-backed anti-narcotics drive, a crackdown denounced by rights activists.

Police said their operation, code-named "Yuktiya" or "Justice", led to the seizure of almost 440 kilograms (970 pounds) of narcotics, including 272 kg of cannabis, 35 kg of hashish, and nine kilograms of heroin. 

Authorities believe the Indian Ocean island is being used as a drug trafficking transit point.

A police statement said 13,666 suspects were arrested while nearly 1,100 addicts were detained and sent for compulsory rehabilitation at a military-run facility.

Local media showed footage of police and soldiers using sniffer dogs to search homes in the capital and elsewhere.

Police said the anti-narcotic raids will pause for the Christmas holidays as officers needed to be deployed on security-related duties, but will resume after Tuesday, which is a Buddhist holiday.

Human rights lawyer Hejaaz Hizbullah said the police raids were illegal as they were conducted without search warrants and urged victims to get the details of officers to initiate legal action later.

Rights activist Ambika Satkunanathan posted on social media that the searches were not based on evidence but were "targeting only poor areas". 

The police were arresting drug users and small-time dealers but "not focusing on large-scale traffickers", she added.

Sri Lanka's biggest drug haul by weight was in December 2016, when police seized 800 kilograms of cocaine.