Hong Kong police fired tear gas to disperse protesters in the upmarket Causeway Bay shopping district on Sunday, after demonstrators had rallied at the US Consulate calling for help in bringing democracy to the Chinese-ruled city.

Police moved on protesters from the Central business district who dispersed to nearby Admiralty, the bar district of Wan Chai and on to Causeway Bay in a now-familiar pattern of cat-and-mouse clashes over three months of unrest.

Activists set barricades, smashed windows, started street fires and vandalised the MTR metro station in Central, the smartest district of the former British colony.

Central district, home to banks, jewellery shops and top-brand shopping arcades, was awash in graffiti, broken glass and bricks torn up from pathways. Protesters set fires from cardboard boxes, building barricades with metal fencing.

"We can't leave because there are riot police," said protesters Oscar, 20, in Causeway Bay. "They fired tear gas from the station. We are heading to North Point."

North Point is east of Causeway Bay.

Thousands of protesters earlier sang the Star-Spangled Banner and called on US President Donald Trump to "liberate" the city. They waved the Stars and Stripes and placards demanding democracy.

"Fight for freedom, stand with Hong Kong," they shouted before handing over petitions at the US Consulate. "Resist Beijing, liberate Hong Kong."

US Defence Secretary Mark Esper on Saturday urged China to exercise restraint in Hong Kong, which returned to Chinese rule in 1997.

Esper made his call in Paris as police in Hong Kong prevented protesters from blocking access to the airport but fired tear gas for a second night running in the densely populated district of Mong Kok.

Last month Trump suggested China should "humanely" settle the problem in Hong Kong before a trade deal is reached with Washington. Earlier Trump called the protests "riots" that were a matter for China to deal with.

The vandalism started in the evening. Police have responded to violence over 14 weeks with water cannon, rubber bullets and tear gas.

Several arrests were made.

Hong Kong returned to China under a "one country, two systems" formula that guarantees freedoms not enjoyed on the mainland. Many Hong Kong residents fear Beijing is eroding that autonomy.

'Fomenting unrest'

China denies the accusation of meddling and says Hong Kong is an internal affair. It has denounced the protests, accusing the United States and Britain of fomenting unrest, and warned of the damage to the economy.

Hong Kong leader Carrie Lam announced concessions this week aimed at ending the protests, including formally scrapping a hugely unpopular extradition bill, which ignited the unrest in June. Many protesters said it was too little, too late.

The bill would have allowed the extradition of people to mainland China to stand trial in courts controlled by the Communist Party. Hong Kong has an independent judiciary dating back to British rule.

But the demonstrations have long since broadened into calls for democracy.

US legislation addressing China's actions in Hong Kong will be among the top priorities pushed by Senate Democrats when Congress returns to work after a recess next week, their leader, Senator Chuck Schumer, said on Thursday.

Schumer urged Senate Majority Leader Mitch McConnell, a Republican who sets the floor agenda, to bring up a bipartisan bill that would require an annual justification of the special treatment afforded by Washington to Hong Kong, including special trade and business privileges, under the US Hong Kong Policy Act of 1992.

The legislation, called the Hong Kong Human Rights and Democracy Act, would also mandate that officials in China and Hong Kong who have undermined the city's autonomy are vulnerable to sanctions.

Protesters, in a petition handed to the US Consulate, urged that it be passed in full.

Joshua Wong, one of the leaders of the pro-democracy "Umbrella" movement five years ago, was re-arrested at the airport on Sunday on return from Germany and the United States for breaching bail conditions, he said.

He had been charged with inciting and participating in an unauthorised assembly outside police headquarters on June 21 and released on bail.

"Preliminary legal advice suggested that the court had acknowledged and approved my trips to Germany and the US when it granted bail on Aug 30," he said in a statement. "Therefore, it is believed that there are some mistakes have been made on the bail certificate."

He said he thought he would be freed on Monday.

- Reuters