Members of the government's human rights commission, accompanied by journalists, found the men and women in an unannounced visit to the station in the heart of Manila's slum area on Thursday evening.
Cries of "here we are, here we are" were heard from behind a wall, according to the rights workers and journalists. The rights workers then found a hidden door behind a bookshelf, leading to the cell.
Stunned detainees came stumbling out of the room, some begging for water while others, in tears, pleaded with the rights workers not to abandon them.
The detainees said they had been held for about a week after being arrested on allegations of drug use or trafficking and that police had demanded hefty payments in exchange for their freedom.
"They were picked up on the pretext of drugs but they had not filed any charges against them," Gilbert Boisner, Manila director for the rights commission who led the inspection, told AFP on Friday.
Human Rights Watch said the incident was another example of widespread rights abuse under Duterte's war on drugs, which has seen thousands of drug suspects killed either by policemen or mysterious vigilantes.
"The discovery of the secret jail is just the latest sign of how police are exploiting Duterte’s abusive anti-drug campaign for personal gain," the group said in a statement.
Duterte briefly suspended all police from the crackdown in January after an official investigation found drug officers kidnapped a South Korean businessman and murdered him as part of an extortion scam.
