Hundreds of Hong Kong pro-democracy protesters faced off against police early Monday in a fresh escalation of tensions, with officers firing pepper spray at angry students trying to surround the government headquarters.

In chaotic scenes, protesters wearing helmets and wielding umbrellas spilled into a major road outside the office of Chief Executive Leung Chun-ying as police tried to beat them back with batons and pepper spray.

"I want true democracy!" protesters yelled. "Surround the headquarters. Paralyse the government."

Protesters have been staging mass sit-ins in Hong Kong for more than two months, demanding free leadership elections for the semi-autonomous Chinese city.

China's communist authorities insist candidates for the 2017 vote must be vetted by a loyalist committee, which the protesters say will ensure the election of a pro-Beijing stooge.

- Bloodied face -

Demonstrators stormed past police lines to occupy Lung Wo Road, a main traffic artery connecting the east and west of Hong Kong island, and hastily erected new barricades from metal railings and plastic cable ties.

Several protesters were injured in the clashes. One was seen led away by police with a bloodied face, while others were tended to by first-aid volunteers after being fired at with pepper spray.

Protesters wore builders' hard hats and used umbrellas -- which have come to symbolise the pro-democracy movement -- to shield themselves from the pepper spray.

Police had to dodge helmets and bottles that were lobbed through the air. One officer was carted into the back of an ambulance on a stretcher.

"I'm more determined than ever, because the police are abusing their power," protester Kelvin Lau told AFP.

"This is a long-awaited escalation of action. It should have happened ages ago."

The protests drew tens of thousands of people at times during their first weeks, but the numbers have dwindled as the movement's leaders struggle to keep up momentum.

Frustrations have grown amongst the demonstrators as Beijing refuses to budge on the vetting of candidates, while support has waned amongst residents grown weary of the transport disruption.

Hundreds of tents continue to block a long stretch of a multi-lane highway outside government headquarters in central Hong Kong, while a smaller camp blocks another busy road in the shopping district of Causeway Bay.

Police cleared a third protest site in working-class Mongkok this week, making more than 140 arrests, but sporadic scuffles there between police and crowds of angry demonstrators have continued.

A British colony until 1997, Hong Kong enjoys civil liberties not seen on the Chinese mainland, including freedom of speech and the right to protest.

But fears have been growing that these freedoms are being eroded.

Sunday's clashes came as a group of British lawmakers investigating Britain's relations with Hong Kong were told China will not allow them into the former colony.

Richard Ottaway, who chairs the cross-party Foreign Affairs Committee, said he would on Monday call for an emergency debate in parliament on the situation.

"The Chinese government have, in past weeks and months, registered their opposition to the inquiry," the committee said in a statement.

The panel is looking into Britain's relations with the Chinese special administrative region 30 years on from the 1984 Joint Declaration, which set out the terms of the 1997 handover of Hong Kong.