Hong Kong student leaders on hunger strike said Tuesday they want to force the government into further talks as the pro-democracy movement finds itself in crisis following violent clashes with police.

Joshua Wong, the teenage face of the movement for political reform, and two young female members of his Scholarism student group went on an "indefinite" hunger strike late Monday, a day after students tried to storm government headquarters.

"We are hoping that after the hunger strike we have a chance to speak with government officials openly -- then there will be a chance to solve this Hong Kong problem," 18-year-old Wong told reporters.

"We would like to put the emphasis on restarting the political reform process," Wong told reporters Tuesday after the trio's first 12 hours of fasting.

The three are carrying out their hunger strike in tents outside government headquarters and were pictured shivering with cold as temperatures dropped in the city.

Embattled Hong Kong leader Leung Chun-ying -- who protesters vilify and want to resign -- urged the students to look after themselves Tuesday.

"I hope the students that are participating in the hunger strike can take care of their health, especially when the weather is getting colder," he told reporters.

Violent clashes broke out Sunday night in a fresh escalation of tensions, with officers firing pepper spray at angry students trying to surround 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.

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 for over two months.

"At this point, we won't decide on whether to retreat or not," Wong said Tuesday.

A court has approved an injunction to clear part of the Admiralty site, but an appeal against it was being heard Tuesday.

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.

Leung warned Monday that the "intolerable" protests will come to nothing and hinted that further police action may take place, in his most forceful comments of recent weeks.

Three student leaders were denied permission to board a flight to Beijing last month where they hoped to bring their demands for free elections to Chinese authorities.

Fruitless talks between demonstrators and the Hong Kong government in October led to an impasse with protest leaders saying authorities had little to offer.

The protests have also sparked a diplomatic row between London and Beijing, with China on Monday defending its refusal to allow a group of British MPs into Hong Kong, describing their planned trip to the former colony as "overtly confrontational" in words echoing those of the legislators' leader.