A Sri Lankan police officer stands guard while colleagues inspect a vehicle at a checkpoint in Colombo. File/AFP

Sri Lanka's police said Sunday they will more than double the number of suspects arrested in an ongoing anti-narcotics crackdown condemned by the United Nations, with tens of thousands already detained.

At least 30,000 people have been arrested since the police operation — code-named “Yuktiya” or "Justice" — began in December, a crackdown denounced by rights activists.

Police on Sunday said they were launching a new phase to arrest a further 42,248 suspects.

"All police stations must work 24 hours a day for the next one month to arrest 42,248 suspects and bring them to justice," the police said in a statement.

It came two days after the UN human rights agency criticised the police crackdown over allegations of unauthorised searches, arbitrary arrests and detention, ill-treatment, public strip searches and torture.

Volker Turk, the UN high commissioner for human rights, urged Sri Lanka's government to review the operation and implement a rights-based approach to combatting illicit drugs.

"Security forces have reportedly conducted raids without search warrants, detaining suspected drug sellers and users, with hundreds sent to military-run rehabilitation centres," a spokeswoman for Turk said Friday.

"While drug use presents a serious challenge to society, a heavy-handed law enforcement approach is not the solution".

Turk's spokeswoman said that lawyers defending detained suspects had faced intimidation from police officers.

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

Sri Lankan police have said nearly 800 kilograms (1,760 pounds) of narcotics, including 340 kilograms of cannabis and 70 kilograms of heroin, have been seized in the operation.

One rights activist, Ambika Satkunanathan, argues the searches are not based on evidence but are "targeting only poor areas" and ignoring large-scale traffickers.

On December 31, police said an officer was killed and another critically wounded when troops opened fire on an undercover unit during a drug raid at a hotel as part of the "Yuktiya" operation.

Agence France-Presse