Chinese police have rescued 33 mentally disabled people forced to work at a building site by slave labour merchants after the apparent suicide of a detainee alerted authorities, a newspaper said on Thursday.

The case in Hulan, a city in the northeastern province of Heilongjiang, echoes a major scandal involving more than 1,000 people forced to work in brutal conditions at brick kilns in Shanxi province last year.

More than 30 captives were discovered crammed in a filthy 30 sq metre (320 sq ft) room in a residential building, the Beijing Times said, after students at a neighbouring police college saw a man who appeared set to jump from a seventh-floor window.

"As the students ran over, the man jumped," the paper said.

They found the captives in a "reeking" room piled with straw mattresses and shabby blankets.

Police later found many of them "could not speak coherently, or clearly remember their names or where their families lived". One had been held for three years, the paper said.

All had been rounded up at a train station in nearby Harbin, capital of Heilongjiang province, and were duped into getting into cars by a gang offering work.

They were sent to do manual labour on a building site, and those that attempted to escape were "beaten and terrorised", the paper said.

Police had detained four suspects and sent 26 of the captives home. The other seven had been found jobs by the local labour bureau, the paper said.

China announced a nationwide crackdown on enslavement and child labour last year after reports of hundreds of poor farmers, children and mentally disabled were forced or lured to work in kilns and mines in Shanxi and neighbouring Henan.