Hong Kong police swooped at dawn to partially clear a democracy protest site Friday, tearing down tents and barricades hours after the city's embattled leader reopened his offer of talks to end nearly three weeks of disruptive demonstrations.

Mongkok has seen violent scuffles between demonstrators calling for full democracy in the southern Chinese city and pro-government groups. 

Hong Kong's embattled leader made a dramatic u-turn Thursday, reopening his offer of talks with student protesters a week after the government abruptly pulled out of discussions aimed at ending more than a fortnight of mass pro-democracy rallies.

The city's Chief Executive Leung Chun-ying made the offer a day after video footage showing plainclothes police officers beating a handcuffed demonstrator sparked widespread anger.

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.

The BBC's website was blocked in China Wednesday, hours after a video of Hong Kong police beating and kicking a pro-democracy protester began circulating online.

The move appears to be the first time the British broadcaster's English-language website has been completely blocked in China since December 2010, when it was inaccessible for days before the Nobel Peace Prize award ceremony for Chinese dissident Liu Xiaobo.

Hong Kong police officers involved in an apparent assault on a protester have been "removed" from their positions, the city's security chief said Wednesday, after video emerged of a handcuffed man being beaten and kicked.

The footage aired by television network TVB after overnight clashes between police and pro-democracy demonstrators sparked outrage and calls for prosecution from activists and lawmakers.

Hong Kong police vowed Tuesday to tear down more street barricades manned by pro-democracy protesters, hours after hundreds of officers armed with chainsaws and bolt-cutters partially cleared two major roads occupied for a fortnight.

In a concerted effort to reduce the territory held by protesters, police tore down barricades in the bustling shopping district of Causeway Bay and on the edge of the main protest encampment in Admiralty, near the city government's headquarters.

Hong Kong's pro-democracy protesters have an "almost zero chance" of changing Beijing's stance and securing free elections despite more than two weeks of rallies, the city's embattled leader Leung Chun-ying said Sunday.

Demonstrators calling for Beijing to grant full democracy to the former British colony have paralyzed parts of Hong Kong, prompting clashes with elements who oppose the blockades and widespread disruption.

A Hong Kong pro-government group said Saturday demonstrators occupying main roads to protest for full democracy would find themselves surrounded if the city's administration failed to clear the barricades.

The Blue Ribbon Movement said the authorities should dismantle three sites the protesters have closed to traffic by Tuesday night or they would encircle them, as the city's political deadlock enters its third week.

Hong Kong protesters are planning a fresh show of force Friday evening after talks with the government dramatically collapsed and U.S. lawmakers urged President Barack Obama to press concerns about the city's democracy fight with Beijing.

Crunch talks between protesters and Beijing-backed city officials were planned for Friday afternoon, but fell apart late Thursday after the government pulled out blaming student leaders for threatening to escalate demonstrations if their demands were not met. 

Hong Kong's government called off talks with pro-democracy students Thursday aimed at ending more than a week of protests and mass sit-ins that have paralyzed parts of the city.

"The basis for constructive dialogue has been undermined. It's impossible to have a constructive meeting tomorrow," Chief Secretary Carrie Lam told reporters.