Hundreds of protesters gathered at the government headquarters in Admiralty last night, as the city geared up for a second wave of protests against a proposed extradition law.

People with multi-coloured umbrellas started streaming to the complex from about 8pm, unfazed by the rain. By 9pm, the walkway in front of the Legislative Council (LegCo) and the bridge next to it were lined with protesters.

Following Sunday's mass protest - the biggest the city has had since the handover in 1997 - there have been calls from activists online for Hong Kong residents to join a second protest, a sit-in slated to begin late last night.

Student Clara Lam, 22, said she attended the mass rally on Sunday and turned up again last night because she is upset that Hong Kong Chief Executive Carrie Lam ignored public opposition to the extradition Bill that many in Hong Kong view as an attempt by Beijing to curb the territory's independence.

"Even if we can't achieve anything, let's take it as the last chance for us to do something about this," she said.

About 5,000 police officers were deployed in anticipation of last night's protest, a local report said. The sit-in comes ahead of a larger rally being planned for today - the day the extradition Bill is due to be debated by lawmakers.

Unions, teachers, students, non-governmental organisations, churches and the Confederation of Trade Unions, among others, have called for a strike.

Already, about 100 small businesses have pledged to close today. More than 40 social welfare and religious groups have also urged employers to let their staff take leave and join the planned rally outside the LegCo.

In an online petition yesterday, hundreds of aviation workers urged airline employees' unions to call for a formal strike, and they demanded that the government immediately withdraw its "evil amendments".

Labour Party legislator Fernando Cheung, a co-organiser of the protest planned for today, had told local media that he expected 2,000 social workers to take part, adding that the groups would ensure there will be emergency services for the needy.

Student unions at seven major higher education institutions have indicated that they will join the protest.

Separately, pro-democracy lawmakers and activists urged people to join them in surrounding the LegCo complex from today until June 20, when the Bill is expected to be voted on.

In anticipation of the protests, officials yesterday cordoned off the demonstration area and Civic Square, and deployed officers to patrol the area. The police also warned they will take action against those who breach the law.

The stern warning follows Sunday night's storming of the LegCo by about 350 protesters. They had stayed behind after the rally ended at 10pm and clashed with the police.

Eight officers and an unknown number of protesters were injured. At least 19 people were arrested.

Meanwhile, Singapore's Consulate-General in Hong Kong yesterday advised Singaporeans to avoid travelling to Admiralty and Wanchai, areas with large crowds and mass gatherings expected in the coming days.