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

Tens of thousands of protesters defied a ban on rallying, the arrests of leading activists and rising threats from China, to take to the streets for a 13th straight weekend.

Police fired round after round of tear gas as demonstrators took cover behind umbrellas between the local headquarters of China's People's Liberation Army and government HQ.

Protesters also threw bricks 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 firebombs. Others shone blue and green lasers at police lines.

Protesters also set fire to a large roadblock near Hong Kong police headquarters.

The group used plastic bollards, metal fencing and seating from a nearby sports ground to build the barricade before starting the blaze in the central district of Wan Chai.

Police had banned the demonstration on security grounds, then organisers had cancelled it, after last weekend saw some of the most violent clashes in months of political turmoil.

But large crowds, many in their signature black T-shirts and under a colourful canopy of umbrellas, snaked through Hong Kong island anyway, blocking roads in the financial heart and chanting "Reclaim Hong Kong. Revolution of our times."

One group passed the official residence of Hong Kong's embattled Beijing-backed leader Carrie Lam, who is the focal point of anger after trying to pass a bill which would have allowed extradition to China.

Another big group massed in the busy shopping district of Causeway Bay, occupying the road and chanting anti-government slogans.

"I'm prepared for the consequences of coming out," said one protestor, who gave his name as Jay, adding "Hong Kongers have the right to assembly." 

Read: What does Hong Kong want? And what will Beijing do about it?

A helicopter buzzed above marchers as police appealed to the public to "leave immediately", raising the prospect of clashes as the day wore on.

Opposition to the extradition bill - now suspended but not permanently withdrawn - has brought much of Hong Kong to the streets.

The protests have expanded into a wider pro-democracy call and a rejection of attempts by Beijing to curtail the freedoms of the semi-autonomous territory.

Police erected barricades near the Liaison Office, the department that represents China's central government, while water-cannon trucks were driven onto the streets in anticipation of renewed clashes with a hardcore minority of the mainly student protesters.

This is the fifth anniversary of Beijing's rejection of a call for universal suffrage for Hong Kong, which sparked the 79-day "Umbrella Movement" in 2014.

Many protesters are determined not to let the new movement fizzle out as the Umbrella protests did.

At least five high-profile activists and three lawmakers were arrested yesterday and rights groups say the tactics are cribbed directly from Beijing's protest playbook.

Protesters taking part in an anti-government rally in central Hong Kong yesterday

Two of the Umbrella Movement's leaders, Joshua Wong and Agnes Chow, were among those arrested, charged and bailed for "inciting others to take part in unauthorised assembly".

The European Union's diplomatic chief, Federica Mogherini, said the developments were "extremely worrying", while US President Donald Trump said his economic pressure on China was forcing Beijing to take a more moderate line in Hong Kong.

In an attempt to sidestep today's protest ban, crowds earlier carried Christian crosses and sang "Hallelujah" in religious gatherings - which do not require the same stringent permission from authorities.

The demonstrators, who have earned a reputation for their creativity and unpredictability, also called for "mass shopping trips" in the city centre.

This morning, LIHKG, the Reddit-like forum used by protesters to communicate, reported via Twitter that its app had suffered the "largest attack it has ever seen".

Police yesterday confirmed three lawmakers - and a district councillor - had been arrested over their alleged actions in previous protests, but denied the sweep was timed specifically to weaken the weekend's protests.

More than 900 people have been arrested since June in connection to protests.

Hong Kong's crisis-hit government has refused to back down over the protests, which have seen millions march peacefully through the streets but also hardcore groups of radical protesters hurl bricks and petrol bombs at police armed with tear gas and rubber bullets.

The violence has damaged Hong Kong's reputation for stability and prosperity.

China has responded with a campaign of intimidation. State media yesterday reported that fresh military anti-riot drills were held across the border in Shenzhen.