Large crowds were beginning to gather in Hong Kong ahead of another mass rally Sunday as public anger seethed following unprecedented clashes between protesters and police over an extradition law, despite a climbdown by the city's embattled leader.

Organisers were hoping for another mammoth turnout rally to keep pressure on chief executive Carrie Lam, who paused work on the hugely divisive bill Saturday after days of mounting pressure, saying she had misjudged the public mood.

An hour before the march was due to start subway stations were filled with dense crowds of black-clad protesters making their way to the start.

Critics fear the Beijing-backed law will tangle people up in China's notoriously opaque and politicised courts and damage the city's reputation as a safe business hub.

Large crowds were beginning to gather in Hong Kong ahead of another mass rally Sunday as public anger seethed following unprecedented clashes between protesters and police over an extradition law, despite a climbdown by the city's embattled leader.

Organisers were hoping for another mammoth turnout rally to keep pressure on chief executive Carrie Lam, who paused work on the hugely divisive bill Saturday after days of mounting pressure, saying she had misjudged the public mood.

An hour before the march was due to start subway stations were filled with dense crowds of black-clad protesters making their way to the start.

Critics fear the Beijing-backed law will tangle people up in China's notoriously opaque and politicised courts and damage the city's reputation as a safe business hub.

Flowers and written tributes were beginning to pile up outside the high-end Pacific Place mall, while demonstrators attending Sunday's rally were urged to bring a flower to pay their respects.

Suspending the bill has done little to defuse simmering public anger and protest organisers have called for a city-wide strike Monday as well as Sunday's rally.

Jimmy Sham, from the main protest group the Civil Human Rights Front, likened Lam's offer to a "knife" that had been plunged into the city.

"Carrie Lam's speech yesterday in no way calmed down public anger," he said.