Thousands of Hong Kong pro-democracy demonstrators have massed outside the offices of the city's leader in a standoff with police, with calls growing for him to resign as protests grip the city.

About 200 riot police stood behind metal barricades as more than 3,000 protesters gathered outside Leung Chun Ying's office in the early hours of Thursday.

"We're trying to surround the government complex and wait for CY (Leung) to come back to work on Friday," protester Thomas Choi told the AFP news agency. "We want to talk to him face to face."

The rally outside Leung's offices comes after three days of peaceful demonstrations where tens of thousands of people have called for Beijing to allow free elections in the semi-autonomous city.

On Wednesday night, one of the student leaders organising the protests threatened to step up the action - including a possible attempt to occupy government offices - if Leung did not resign by Thursday.

Leung has not agreed to the protesters demands.

"We will consider having different operating actions in future days, including occupying other places like important government offices," said Agnes Chow of student movement Scholarism.

'We need to escalate the movement'

There was a mixed response to the call for an escalation of action.

"We need to escalate the movement," said 23-year-old student Jason Chan. "So many people have come out every day and the government hasn't responded to us. If we don't take things to the next level, this movement is pointless."

But many others were reluctant to take any action that could prompt further clashes with the police.

"I think we should keep this a peaceful revolution," said costume designer Janice Pang. "Hong Kong people may not support us if we do something more extreme."

The protesters are angry at the central government's refusal to allow free elections for the city's next leader in 2017, insisting that only two or three candidates vetted by a pro-Beijing committee will be permitted to stand.

They call this "fake democracy" and have two demands - that Leung steps down and that Beijing reverses its decision.

Source: AFP