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

About 200 police equipped with riot helmets, gas masks and shields stood firm behind metal barricades in the early hours of Thursday as more than 3000 protesters gathered outside Leung Chun-ying’s office.

A car nearly hit the crowd as it swerved past, further raising tensions as the protesters yelled at the police from behind the barricades and chanted for Leung – seen by critics as a Beijing stooge – to step down.

The spike in tensions came after three days of peaceful demonstrations in which tens of thousands of people took over major Hong Kong highways, calling for Beijing to allow free elections in the semi-autonomous city.

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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.

“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.

Students have been at the forefront of the demonstrations but others have swelled their ranks since riot police tear-gassed protesters on Sunday night, in chaotic scenes that triggered an outpouring of support.

The protesters are furious 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.

In a movement being dubbed the ‘umbrella revolution’ – a nod to the umbrellas they have used to protect themselves against tear gas, the sun and torrential downpours alike – the protesters have brought key parts of the city to a standstill, heavily disrupting the transport network and shutting down businesses.

Wednesday and Thursday are both public holidays in Hong Kong, leaving many workers free to swell the masses in the streets.

China on Wednesday issued a stern warning to Washington to stay out of the situation, brushing off US calls for restraint and saying the protests were none of its business.

However, support for the protesters has been growing worldwide, with smaller “solidarity” demonstrations held from Taipei and Singapore to London and Washington.