The captives were discovered during a raid by the Commission on Human Rights (CHR), following a tip-off.

The undocumented prisoners said they had been there for a week, and alleged police at the Manila station had locked them up in order to extort money.

But officers said they were arrested as part of President Rodrigo Duterte's war on drugs.

Mr Duterte has launched a brutal crackdown across the country, leading to the deaths of more than 7,000 suspected dealers, killed by police or vigilante groups.

Tens of thousands have also been arrested since the it was launched last year.

Superintendent Robert Domingo told local media the captives discovered on Thursday had been rounded up during a "one-time, big-time" raid, and had no documents because they were still being processed.

Image copyright EPA

Image caption Philippine president Rodrigo Duterte has encouraged a brutal crackdown on drugs

But local media - who appear to have been invited along to film the CHR raid - reported that many of the people inside the windowless room said they were innocent, and were being asked to pay between P40,000 and P200,000 ($800-$3,990/£609-£3,095) for their release, according to figures released by ABS-CBN.

They also made allegations of torture, and told how the room had no electricity and they were forced to use plastic bags instead of a toilet.

Supt Domingo denied all the allegations, telling The Inquirer that the 1m x 3m room was "such a waste of space".

"Why should I not maximize it?" he added.

CHR spokesperson Jackie De Guia said they would continue to investigate what had happened in the police station, noting to GMA Network: "No normal detention facility is kept behind equipment such as bookshelves."