The heart of Hong Kong Island again resembled a war zone on Sunday as rioters went on the rampage, fighting fierce running street battles with the police, smashing up and vandalizing MTR stations, committing arson and assaulting members of the public.

More than 20 people were injured, one seriously, in Sunday's fresh wave of violence that has rocked the city for nearly four months, following similar clashes between police and radical demonstrators the previous day.

The HKSAR government issued a strong condemnation of the reckless behavior of the radical protesters, saying their display of violence, in total disregard of the rule of law, had seriously put public safety at risk.

Four MTR stations were forced to shut down on Sunday as streets in Wan Chai and the popular shopping district of Causeway Bay burned, with rioters resorting to their usual pattern of setting fire to makeshift barricades, hurling gasoline bombs and bricks at the police. Thick smoke billowed from the entrances of metro stations, while the police retaliated with water cannons, tear gas and bean bag rounds.

Almost all shops and major shopping malls in Wan Chai and Causeway Bay pulled down shutters on Sunday after an assembly by rioters went ahead without police authorization.

Rioters were seen waving flags of the United States and several other countries as they marched from Causeway Bay to the Central Government Offices in Admiralty.

During the clashes, some radical protesters charged at police lines and flung gasoline bombs and bricks at police officers at Admiralty, while setting fires at several locations. A footbridge at Admiralty was seen engulfed in smoke.

Two upmarket shopping malls, Sogo and that at World Trade Centre in Causeway Bay suspended business throughout Sunday, while another shopping landmark, Hysan Place, closed at 1 pm, followed by most other stores at Times Square.

An outlet of fast-food restaurant chain Cafe De Coral and a Starbucks shop on Hennessy Road were vandalized by rioters, with offensive graffiti scrawled at their doors and on logos.

Wan Chai MTR station was again targeted as radicals smashed one of its entrances and tossed gasoline bombs into it.

Four metro stations along the Hong Kong Island Line — Admiralty, Wan Chai, Causeway Bay and Tin Hau — had to close from 6:30 pm as the violence intensified.

MTR Corp — the city's sole railway operator — said some of the vandalized stations may not resume services on Monday as their facilities had been severely damaged.

As the chaos escalated, a man wearing a press vest was struck by a Molotov cocktail tossed by a masked rioter, while another demonstrator was seen attempting to throw a natural gas tank into a fire on a Wan Chai street, but was stopped by his companions.

Another man thought to have criticized the rioters was beaten by a group of black-clad rioters with fists, feet and umbrellas. On Saturday, a man displaying a Chinese national flag during a protest at Tamar Park was brutally attacked by a group of rioters. His face was seen covered in blood.

Similar attacks on members of the public had occurred during protests in the past few weekends. Tse Chun-chung, chief superintendent of the Police Public Relations Branch, said last week lynching by rioters has become common and getting out of hand.

The Hong Kong Federation of Trade Unions — the city's largest labor group — condemned the latest violence after it was forced to cancel a cultural gala scheduled to be held on Sunday evening to celebrate National Day.

Federation Chairman Wong Kwok said the rioters had breached the "one country, two systems" bottom line and blatantly challenged national sovereignty.