Hong Kong pro-democracy protest leaders pledged to stay on the streets Wednesday in a bid to keep pressure on the government ahead of already fraught talks on political reform.

Numbers at demonstration sites around the city have dwindled to a few hundred after days of mass rallies to demand fully free elections and police said Wednesday they were employing negotiators to try to persuade protesters to leave the barricades.

But student leader Lester Shum insisted they would stay.

"We have reached the stage of dialogue and we will be persistent in our civil disobedience campaign and stay on the streets," he said.

Leader of the pro-democracy Occupy movement, Chan Kin-man, added: "Only if the government quickly take action and give a concrete response to students' demands (for democracy) will the problem be resolved."

Formal talks are set for Friday between students and Chief Secretary Carrie Lam -- the deputy to Hong Kong's embattled leader Leung Chun-ying.

Pro-democracy leaders had agreed to a dialogue earlier with Lam but called it off last Friday after what they described as "organised attacks" on protesters at the Mong Kok demonstration site.

"We are very sincere about organising this meeting and dialogue," said Ray Lau, deputy secretary for mainland and constitutional affairs.

But he added there were "many difficulties ahead", with disagreements over arrangements, including the venue and whether there should be a mediator.

The authorities have given no indication of concrete plans to clear the protest sites and reopen roads, despite traffic gridlock and truncated bus routes.

"(With) all the obstacles on the road, we have already arranged the police negotiators and the police community relations officers to engage in dialogue with people gathering there," Lee Kwok-chung, senior superintendent of traffic police, told reporters.

"We tried to convince them to remove the blockage as soon as possible and, if not, we will continue to monitor the situation and take suitable actions if needed."

- 'Hold on regardless' -

Protesters are demanding "true democracy" after Beijing insisted that it vet candidates for the city's 2017 leadership elections.

Rising social inequality, the soaring cost of living and distrust of the government is also fuelling discontent, particularly among Hong Kong's disillusioned youth.

Protesters remain sceptical that talks with the government will achieve any concessions on political reform and those who remain on the street want to stand their ground.

"This is an important spot -- we should hold on to this regardless of how many people we have," said Helix Kwok, an 18-year-old student who was among a dozen protesters camping outside the Chief Executive's office Wednesday.

"If we withdraw, the government will then ignore us. With us here, we can react to anything that happens."

The government complex has been one of the key protest sites and scene of tense standoffs between police and students, with Wednesday's opening of parliament called off over security fears.

"I'm not very hopeful about the outcome (of talks)," said Timothy Sun, 17, who has spent 10 days at the main Admiralty protest site.

"I think the government is going to repeat the same thing as before -- instead of accepting our requests such as civil nomination, they say that we'll have free elections step-by-step.

"I am going to stay on. I don't want this protest to be a failure."

Demonstrators are facing an increasing public backlash as road diversions due to the occupied sites jam the highways and businesses say they are suffering.