Joshua Wong, the teenage face of Hong Kong's pro-democracy movement, and two other student demonstrators went on hunger strike Monday, raising the stakes after one of the worst nights of violence to hit the demonstrations.

Wong, 18, and two young female members of his Scholarism student group announced the "indefinite" hunger strike hours after Hong Kong's leader warned that the two-month-old protests are "in vain".

Student-led demonstrators are demanding free leadership elections for the semi-autonomous Chinese city, with the main protest camp continuing to block a long stretch of a multi-lane highway in central Hong Kong.

China's communist authorities insist that 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.

With frustrations mounting, violent clashes broke out Sunday night in a fresh escalation of tensions, with officers firing pepper spray at angry students trying to surround the government headquarters.

Civil servants were forced to stay at home on Monday morning and the city's legislature was suspended after protesters broke through police lines and occupied a major road outside the complex overnight.

Wong, one of the most prominent faces of the so-called "umbrella movement", said he was launching a hunger strike alongside university student Isabella Lo, 18, and 17-year-old high school pupil Prince Wong in a bid to force the government to respond to their demands.

"Living in these troubled times, there is a duty. Today we are willing to pay the price, we are willing to take responsibility," the students wrote on Facebook after announcing the strike onstage at the main protest camp.

"Our future, we will take it back."

Wong, who is in the running to be named TIME magazine's person of the year, called on Hong Kong authorities to reopen stalled talks with students and for Beijing to withdraw its decision to vet candidates for the vote.

But the announcement came just hours after Hong Kong's leader Leung Chun-ying warned that the "intolerable" protests will come to nothing and hinted that further police action may be imminent, in his most forceful comments of recent weeks.

"Now the (public) demand for police clearance is increasing. From now on, police will enforce the law without hesitation," Leung told reporters.

On Monday the high court granted an injunction ordering the clearance of several parts of the main protest camp in the Admiralty district, according to bus operator Kwoon Chung, which made the application.

A smaller camp continues to block another busy road in the shopping district of Causeway Bay.

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

- 'Angry and tired' -

There was frustration and pessimism in Admiralty on Monday following the clashes.

"We feel a mixture of things: angry, tired, upset. All the emotions are quite negative and tense," said student Eppie Chan.

Police arrested 40 people and 11 officers were injured, a spokesman said. A total of 37 people received hospital treatment.

The Admiralty site had calmed by Monday afternoon after a chaotic morning which saw protesters clash with police inside a nearby shopping arcade.

The protests drew tens of thousands of people onto the streets in their first weeks. But numbers have dwindled after two months without progress, and support has waned among residents weary of the transport disruption.

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 under Chinese rule, while frustrations have been building over growing inequality in the freewheeling financial hub.

In London, a senior lawmaker denied that Britain was behaving like an interfering colonial power, after China barred his parliamentary committee from visiting Hong Kong.

The Foreign Affairs Committee is looking into Britain's relations with the Chinese special administrative region 30 years after the 1984 Joint Declaration, which set out the terms of the city's handover.

Chinese foreign ministry spokesman Hua Chunying said Monday that foreign countries had no right to "interfere" in Hong Kong, and called the politicians' attempt to travel "overtly confrontational".

But committee chairman Richard Ottaway said the lawmakers had "every right" to visit.

"I don't think for a moment that we think that we're still a colonial power," he told AFP.

"We've got every right to ascertain whether China is complying with its undertakings."