Two pro-democracy protesters have been arrested as they emerged from a manhole on a Hong Kong road outside a besieged university, in a thwarted escape bid from inside the campus.

Key points: Several protesters involved in the attempted escape were arrested for "taking part in a riot"

Several protesters involved in the attempted escape were arrested for "taking part in a riot" Dozens of protesters are believed to still be inside the university

Dozens of protesters are believed to still be inside the university US Congress approves new bills that support protesters and human rights

The attempted escapes took place hours before the United States Congress approved two bills that support protesters and human rights, and will ban US companies from selling tear gas, rubber bullets and other defence items to Hong Kong police.

Two male protesters holed up in the campus for days were detained along with four people lowering ropes to help them out from the drainage system, police said.

They reportedly crawled through sewers to a manhole outside a housing estate about 500 metres from the grounds of the Hong Kong Polytechnic University (PolyU), the scene of the dramatic siege by police.

The group was arrested for offences including "taking part in a riot" and "assisting offenders," chief superintendent Ricky Ho said.

Dozens of pro-democracy protesters are believed to still be inside PolyU but are running out of supplies.

Some have made successful and daring escapes — climbing down ropes and jumping on waiting motorbikes — or making a run for it during the night.

The city's fire service said that "no person has been found trapped inside the [sewer] system".

The siege at Hong Kong Polytechnic University is part of the ongoing pro-democracy protests. ( Reuters: Tyrone Siu )

Many others have been arrested while trying to breach police lines, with some being beaten with batons and kicked in the head by police.

Dozens of those who made the Monday night breakout by ropes off a bridge were subsequently arrested, police said.

Hong Kong has been rocked by nearly six months of demonstrations against the city's Beijing-backed government over what they say are decreasing freedoms.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 40 seconds 1 m 40 s Students besieged inside Hong Kong Polytechnic University tried to unsuccessfully escape the campus through sewers

US Congress approves support for Hong Kong

The US House of Representatives approved the bills the Senate passed days earlier, with President Donald Trump expected to sign both into law.

The bills, which have angered Beijing, would require the State Department to certify at least once a year that Hong Kong retains enough autonomy to qualify for the special US trading consideration that helped it become a world financial centre.

It also would provide for sanctions against officials responsible for human rights violations in the city.

The second bill bans the export of crowd-control devices to Hong Kong police forces, including tear gas, pepper spray, rubber bullets and stun guns.

Some protesters have escaped by climbing down ropes and jumping onto waiting motorcycles. ( AFP: Anthony Wallace )

AFP/Reuters