Hong Kong police reopened public access to a university campus on Friday after blocking it for 12 days to try to arrest anti-government protesters holed up inside.

A cordon around the university has been lifted and investigators have left Hong Kong Polytechnic University, a statement posted on the police Facebook page said.

About 100 officers first entered the campus on Thursday to collect evidence and remove dangerous items. The statement said that over two days, they seized 3,989 gasoline bombs, 1,339 explosive items, 601 bottles of corrosive liquids and 573 weapons.

Police did not encounter any protesters. One masked protester told media the night before the police came in that about 20 people were still hiding to avoid arrest.

They were the holdouts from roughly 1,000 protesters who had retreated inside the campus after battling police on nearby streets. A few escaped a police cordon, but police say they arrested 700 people and recorded the details of 300 minors who could face charges later.

The standoff at the university was one of the most dramatic episodes in six months of unrest that began in June with protests over an extradition bill seen as an erosion of freedoms promised when the former British colony returned to Chinese control in 1997.

Protesters have since expanded their demands to include fully democratic elections and an investigation into police use of tear gas, rubber bullets and other force to suppress the demonstrations. More than 5,000 people have been detained since June.

In Thailand, city leader Carrie Lam said that Hong Kong is undergoing a difficult period but that the fundamentals of the Asian financial centre, including its strengths under the one country, two systems framework, remain strong.

"I and my government are listening to our people with a view to resolving some deep-seated problems in Hong Kong through dialogue," she said. "I have every confidence that Hong Kong can bounce back, as we always do."

Thousands of people rallied in Hong Kong on Thursday night to thank America, on its Thanksgiving Day holiday, after United States President Donald Trump signed into law two bills supporting the protesters.

Chanting fight for freedom, stand with Hong Kong, they urged the world to join the US in supporting human rights in the semi-autonomous Chinese territory.

Waving US flags, they vowed to continue with their fight.

Rally organiser Ventus Lau urged the US to implement the laws swiftly and penalise police and government officials who suppressed democracy.

One of the laws prescribes sanctions on officials found guilty of human rights abuses and requires an annual review of a special trade status for Hong Kong. The other bans the export of certain nonlethal munitions to Hong Kong police.

Joshua Wong, an activist who went to Washington to lobby for the new US laws, said it was remarkable that human rights had triumphed over crucial US-China trade talks.

In Beijing, China called in US Ambassador Terry Branstad on Thursday and told him that the move constituted serious interference in China's internal affairs and a serious violation of international law.

Hong Kong government also denounced the US legislation as unreasonable meddling, saying it sends the wrong signal to protesters and won't help to ease the crisis.