The United States has called for a "swift, transparent and complete" investigation into the beating of a handcuffed Hong Kong democracy protester by plainclothes police, as fresh street clashes broke out early Thursday.

Television footage of officers assaulting the unarmed protester in a dark corner of a public park has sparked outrage and calls for prosecution from activists and lawmakers in the city.

Tensions soared in the former British colony after the video went viral Wednesday, with protesters saying they had lost all faith in the police despite the accused officers being "removed" from their posts by city authorities.

The United States said it was "deeply concerned" by the police brutality reports.

"We encourage Hong Kong authorities to carry out a swift, transparent, and complete investigation into the incident," State Department spokeswoman Jen Psaki told reporters.

"We renew our call for the Hong Kong government to show restraint, and for protesters to continue to express their views peacefully."

The Asian financial hub has been rocked by mass rallies for much of the last fortnight calling for full democracy and causing significant disruption to a city usually known for its stability.

Protesters are angry at China's insistence that it vet candidates standing for election as the city's next leader in 2017, a proposal they have dubbed "fake democracy".

After weeks of largely ceding control to protesters at three main sites, police have begun probing demonstrator defences in the last few days, tearing down some barricades and sparking running battles.

But the city authorities face a bigger backlash after footage was aired showing officers hauling a handcuffed protester to the quiet corner of a protest-hit public park, placing him on the ground and beating him with kicks and punches.

"It is stomach-churning to think there are Hong Kong police officers that feel they are above the law," Mabel Au, director of Amnesty Hong Kong, said in a statement.

The incident has become another public relations disaster for the police, who were severely criticised for firing tear gas on umbrella-wielding protesters on September 28 in a move that attracted worldwide attention.

Demonstrators have also accused officers of failing to come to their aid during several attacks by violent pro-government thugs.

Prime Minister David Cameron meanwhile said Britain would stand up for Hong Kong's rights and freedoms, "including those of person, of speech, of the press, of assembly", while his spokesman later urged police and protesters for restraint.

- Fresh clashes -

With trust between the government and protesters at an all time low, renewed clashes between police and protesters broke out again in the early hours of Thursday over a contested road near Hong Kong leader Leung Chun-Ying's offices.

Officers used pepper-spray against defiant demonstrators shouting chants accusing them of links with criminal triad gangs -- but the flurry of violence was brief compared to the night before.

Several thousand gathered late Wednesday at the main protest site in Admiralty to hear speeches by protest leaders, who urged demonstrators to stay peaceful in the face of violence.

"The major thing is people have realised how police become corrupt under a government that doesn't represent the people of Hong Kong," said Kay Wong, 25, a university research assistant.

"I was shocked at the police violence last night. Who wouldn't be?"

Police said seven officers had been identified in relation to the video and that they would be reassigned pending an investigation.

Hong Kong's justice chief insisted Wednesday that any prosecution of plainclothes officers who were filmed beating a handcuffed protester would be handled impartially, while security chief Lai Tung-kwok sought promised a "just and fair investigation", without specifying how many officers were being probed.

Following Wednesday's clashes, a Beijing official said China saw no need "so far" for its army to be deployed to contain the protests.

Rumours have frequently swept protest camps that the People's Liberation Army, which maintains a garrison in the city, will be deployed if Beijing feels Hong Kong police cannot handle the demonstrations.

Hong Kong's richest man, Li Ka-shing, urged protesters to leave the streets, saying they had got their message across.

Protests were largely peaceful until the clashes early Wednesday. Although ugly scuffles have broken out between demonstrators and government loyalists, sparking accusations that the authorities are using hired gangsters.

Patience with protesters is also running short in some quarters, with shop owners and taxi drivers losing business and commuters voicing irritation at disruptions and delays.