Pro-democracy protesters hold umbrellas during a protest against police violence during previous marches, near China's Liaison Office, Hong Kong, China July 28, 2019.

Hong Kong police clashed with thousands of protesters on Sunday, as they sought to defend China's main representative office from crowds seething over what many see as an increasing cycle of violence against them.

Protests over the past two months spearheaded by anti-government activists against a proposed bill that would allow people to be extradited from the city to stand trial in courts in mainland China have grown increasingly violent.

A march on Saturday against an assault the previous weekend by suspected triad gang members ended in violent turmoil as riot police waded in to disperse crowds.

On Sunday, a peaceful gathering in a park in the city's central business district rapidly morphed into a march, as tens of thousands of black-clad protesters set off in several directions, clogging up major thoroughfares.

Thousands of people headed east, towards the shopping district of Causeway Bay, while another large contingent headed west, towards the Chinese government's representative office, known as the Central Government Liaison Office.

There, hundreds of riot police blocked activists from advancing towards the building, which had been heavily fortified with barricades after it was surrounded and defaced a week earlier. A clear plastic shield had been erected around a national emblem above its front doors.

As the crowds surged, hundreds of riot police with shields advanced, firing rounds of tear gas, rubber bullets and sponge grenades - a crowd-control weapon - at protesters, sending clouds of acrid, burning smoke through the streets.

Some protesters were on their knees choking as ambulances raced to take away the injured.

The mostly young activists in hard hats, gas masks and body armour dug in, dismantling street signs and fences which they used to form makeshift barricades to slow police advances.

Many hit metallic surfaces with sticks to create an ominous drum beat that echoed down the streets.