Hong Kong's student leaders said Thursday they would decide in the coming days whether to leave protest sites they have occupied for more than two months, following violent clashes.

The rallies for fully free leadership elections drew tens of thousands at their height, but numbers have dwindled as public support for the movement has waned.

One prominent protest leader said the students would decide "within a week" whether to leave two remaining camps in the centre of the southern Chinese city after authorities cleared a third last week.

Their announcement came after the three leaders of protest group Occupy Central handed themselves in to police Wednesday in a symbolic move to get demonstrators off the streets, after violent confrontations with police outside government headquarters at the weekend.

China insists that candidates for the vote for chief executive in 2017 must be vetted by a loyalist committee, which demonstrators say will ensure the election of a pro-Beijing stooge.

Student protest leaders have remained adamant that staying on the streets is their only option to force reform.

But they said Thursday that the decision to retreat was now an option.

"There needs to be a decision that is made about whether to leave or stay," said Yvonne Leung, of the Hong Kong Federation of Students, which has spearheaded the mass street protests.

"Within a week's time, we definitely will have to have a decision," she told local radio.

Leung said the main reason for considering a retreat was the police violence on Sunday that left dozens injured, including police officers, as well as public pressure for protesters to leave.

- Hunger strike continues -

Protesters poured onto the streets on September 28 demanding political reforms, blocking three major junctions across the financial hub.

One site, in the busy commercial district of Mongkok, has since been cleared by police.

But the main Admiralty camp outside government headquarters in central Hong Kong still takes up a long stretch of a multi-lane highway through the heart of the business district.

A third smaller site blocks a busy road in the Causeway Bay shopping area.

Tommy Cheung, also of HKFS, told reporters the group would consider tactics "including whether to retreat or to stay" in the next few days.

Teenage protest leader Joshua Wong of the Scholarism group, also at the forefront of the street demonstrations, said they would work with HKFS on the next step.

"Internal discussions will be made and we will stay in touch with HKFS, exchange ideas and coordinate," he said in a Facebook post.

"I hope that we do not put the focus on whether to retreat, but focus on whether the government will talk."

Wong began a hunger strike on Monday night in a bid to force new dialogue with the government over political reform and has been joined by four other student protesters.

He vowed Thursday to continue, despite city leader Leung Chun-ying rejecting the hunger strikers' call to relaunch talks, adding that he felt "dizzy".

"My sugar level dropped to a low level," Wong said, speaking near the tent outside government headquarters where the hunger strikers are sleeping.

Occupy Central leaders Benny Tai, Chan Kin-man and Chu Yiu-ming were released by police Wednesday after turning themselves in for "participating in unauthorised assembly".

The police said they would conduct follow-up investigations.

Analysts say the move is a bid to take the moral high ground and engender renewed public sympathy -- the trio have encouraged other supporters to give themselves up as a sign of their commitment to the rule of law.

Police said 65 people had handed themselves by Wednesday night but did not immediately have a figure for Thursday.

Washington meanwhile gave its strongest backing yet to the protesters, calling for "competitive" leadership elections for the semi-autonomous Chinese city.

"The legitimacy of Hong Kong's chief executive will be greatly enhanced if the promise of universal suffrage is fulfilled," the top US diplomat for Asia, Daniel Russel, told US lawmakers.

"This means allowing for a competitive election in which a range of candidates with differing policy approaches are given an opportunity to seek the support of eligible Hong Kong voters."