Hong Kong police fired tear gas and water cannon on Saturday as pro-democracy protesters threw petrol bombs in the latest in a series of clashes that have plunged the Chinese-ruled city into its worst political crisis in decades.

Police fired round after round of tear gas as protesters took cover behind umbrellas between the local headquarters of China's People's Liberation Army (PLA) and government HQ. Protesters also threw bricks dug up from pathways, at police.

The water cannon fired blue-dyed water, traditionally used elsewhere in the world to make it easier for police to identify protesters later.

Riot police then marched on foot towards the neighbouring Admiralty district, followed by about 20 police cars, where some protesters had thrown fire bombs from flyovers, some landing close to police. Others shone blue and green lasers at police lines.

There were no immediate reports of injuries.

The protests, which at one point blocked three key roads, came on the fifth anniversary of a decision by China to curtail democratic reforms in the former British colony, which returned to China in 1997.

The PLA on Thursday rotated its troops in Hong Kong in what it said was a routine operation. Their Hong Kong HQ was the former base of the British military garrison.

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Thousands took to the streets in the afternoon on a largely peaceful, meandering rally in the rain, many joining a "Christian march" from the Wanchai entertainment district and congregating next to the Legislative Council, stormed by activists in an earlier protest.

Other demonstrators, many wearing black and face masks, marched in the Causeway Bay shopping district. The crowds grew after dusk in Wanchai, where protesters built roadblocks and banged iron sticks. Firemen battled a huge fire outside a Methodist church in the main Hennessy Road.

The protests have gone on for three months, sometimes turning violent, and have targeted the airport, the legislature and the Liaison Office, the symbol of Chinese rule.

"Hong Kong has religious freedom," said Sally Yeung, 27, a Christian. "We are praying at different check points and praying for justice to arrive in Hong Kong. If they prosecute us simply because we are praying, they infringe our religious freedom.”

Hovering under an umbrella outside the government offices, Eric, a 22-year-old student, said telling people not to protest was like telling them not to breathe.

"I feel it's my duty to fight for democracy," he said. "Maybe we win, maybe we lose. But we fight."

Police erected water-filled plastic barriers around key government buildings and two water cannon, used briefly for the first time last weekend, were at the ready near the Liaison Office, which was daubed with graffiti in an earlier protest.

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The MTR subway suspended some services and shut station exits because of likely "public activities". Police said protesters vandalised the legislature.

'Be like water'

Police arrested a number of prominent pro-democracy activists and three lawmakers on Friday, seeking to rein in a movement which started with anger over planned legislation allowing extraditions to mainland China. It soon broadened into calls for democracy amid fears China is squeezing Hong Kong's freedoms.

But the latest protests have no leaders. The slogan is "be like water", meaning be flexible. Marchers on Saturday were marching here and there, wherever streets took them, communicating with different hand signals and chanting "stand with Hong Kong" and "fight for freedom".

China denies the charge of meddling in Hong Kong, which it says is an internal affair. It has denounced the protests and warned of the damage to the economy.

China is eager to quell the unrest before the 70th anniversary of the founding of the People's Republic of China on October 1 but protesters vandalised a long red banner celebrating the event to cheers from the crowd.

Beijing has also accused foreign powers, particularly the United States and Britain, of fomenting the demonstrations and warned against foreign interference.

Hong Kong returned to China under a "one country, two systems" formula that allows it to keep freedoms not enjoyed on the mainland, like the freedom to protest and an independent legal system.

There have been frequent clashes between protesters and police, who have often fired tear gas and rubber bullets to disperse crowds, amid accusations of excessive force.

"A lot of people from the outside think it is the police who escalate [the violence] first," a police officer told a media briefing. "This is not true."

An off-duty policeman was attacked late on Friday night by three unidentified men with a knife in the Kwai Chung container port area, suffering wounds to his limbs and back, police said. The news was a top-trending topic on China's Twitter-like Weibo.

With protesters and authorities locked in an impasse and Hong Kong facing its first recession in a decade, speculation has grown that the city government may impose emergency laws, giving it extra powers over detentions, censorship and curfews.

Lawmaker Fernando Cheung said the arrests of the three legislators were probably aimed at causing more anger and chaos to justify the use of emergency laws.

"To incite more people to come out today is totally ridiculous," he told Reuters.