Police in Sri Lanka said the arrests were made as a part of the operation Yuktiya (the Sinhala word for justice) which began in mid-December and aims to completely eradicate the illegal drug trade in the South Asian nation.
Law enforcement officials have been accused of subjecting the suspects to unauthorised raids, arbitrary arrests, ill-treatment, public strip searches and torture.
The UN human rights agency slammed the operation on Saturday and called on the government to reassess its strategy with a human rights-based approach.
“We are very concerned that authorities in Sri Lanka are adopting a heavily security-based response to the country’s drugs problem," read a statement issued by Volker Turk, the UN high commissioner for human rights.
More than 29,000 people have been arrested between the start of the operation on 17 December and the first week of January on suspicion of drug-related offences, with the police vowing to detain more. About 1,500 of those detained were in administrative detention in police custody, while 1,600 were sent to compulsory drug rehabilitation centres.
"Abuse of drugs and the factors that lead to it are first and foremost public health and social issues," a spokesperson for Mr Turk said. "People suspected of selling or trafficking drugs are entitled to humane treatment, with full respect for due process and transparent, fair trials."
Human Rights Watch, Amnesty International and a group of 30 rights organisations on Monday urged the government to cease the crackdown and release any people arrested without reasonable evidence.
Suspects with a history of drug abuse were being arrested without evidence, the group said, adding that the detention adversely impacted families’ ability to meet their basic needs at a time of economic crisis.
"This operation is unfolding in a context of already severe repression against persons who use or are suspected of using drugs, who suffer discrimination and stigma within the Sri Lankan criminal justice system and society," the group said.
"Cease involving the armed forces in drug control and treatment activities as consistent with human rights law," it said.
Despite the international concerns, the Sri Lanka police on Sunday said that they would launch a new phase of the drug bust operation to arrest over 40,000 identified suspects before the June deadline.
"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.
Nearly 800kg of contraband, including 340kg of cannabis and 70kg of heroin, have been seized during the operation, it added.