Hong Kong braced for mass strikes on Wednesday - its second since Sunday's "million march" - as the city's Legislative Council prepares to debate a controversial extradition bill that would allow people to be sent to mainland China for trial.

Security has been tightened in and around the legislature building, with riot police deployed in some areas, as thousands of people are expected to join the protesters on Wednesday as businesses across the semi-autonomous Chinese territory prepared to go on strike.

Hong Kong leader refuses to scrap extradition bill despite rally

The bill, which has generated unusually broad opposition at home and abroad, is due for a second round of debate on Wednesday in Hong Kong's 70-seat Legislative Council. The legislature is controlled by a pro-Beijing majority.

A final vote is expected on June 20.

"When the fugitive extradition bill is passed, Hong Kong will become a 'useless Hong Kong'," said Jimmy Sham, convenor of Civil Human Rights Front, the main organiser of Sunday's demonstration.

"We will be deep in a place where foreign investors are afraid to invest and tourists are afraid to go. Once the 'Pearl of the Orient', (it) will become nothing."

Deep concerns

Hong Kong Chief Executive Carrie Lam said she would press ahead with the legislation despite deep concerns across large swathes of the Asian financial hub that on Sunday triggered its biggest political demonstration since its handover from British to Chinese rule in 1997.

Lam has sought to soothe public concerns and said her administration was creating additional amendments to the bill, including safeguarding human rights.

Sunday's protest, which organisers said saw more than a million people take to the streets, in addition to a snowballing backlash against the extradition bill could raise questions about Lam's ability to govern effectively.

Protesters remained defiant in Wednesday's early hours, rallying peacefully just a stone's throw from the heart of the financial centre where glittering skyscrapers house the offices of some of the world's biggest companies, including HSBC.

"I want to do something before our freedoms are taken away," said Yu Wing-sum, 23.

Protesters stood under umbrellas in heavy rain, some singing "Hallelujah", as police conducted random ID checks.

Plainclothes officers, in jeans and sneakers and carrying batons and shields, were also deployed, with other reinforcements gathering behind barricades.

Sunday's peaceful march had ended up in violence with officers fighting running battles with small groups of hardline protesters.

The Civil Human Rights Front condemned the searches, saying authorities had made people afraid to participate in peaceful gatherings.