Tens of thousands of pro-democracy protesters flooded Hong Kong's financial and shopping districts Saturday, defying a police ban and a sweeping crackdown which saw several key activists and lawmakers arrested.

Police banned Saturday's planned demonstration on security grounds, after last weekend saw some of the most violent clashes in three months of political turmoil.

But by early afternoon, thousands of protesters many in their signature black T-shirts and under a colourful canopy of umbrellas, snaked through Hong Kong island, blocking roads.

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

Another large group massed in the famed 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.

"As Hong Kongers we have the right to assembly."

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Opposition to the extradition bill, now suspended but not permanently withdrawn, has brought much of Hong Kong to the streets.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 since expanded into a wider pro-democracy call and a rejection of attempts by Beijing to curtail the freedoms of the semi-autonomous territory.

"It's 'now or never' for Hong Kong, " said a 33-year-old accountant who gave her surname as Wong.

"I'm a mother-of-two. They didn't come today but their grandmother did. We're defending the right of assembly for the next generation in Hong Kong."

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.

Saturday marks 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 on Friday in a sweep aimed at defanging Saturday's rally.

Rights groups say the tactics are cribbed directly from Beijing's protest playbook.

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.

Protesters sidestep ban

In an attempt to sidestep Saturday'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.

"The mass arrests yesterday were an obvious tactic to scare us off. Instead, (the government) lures us to come out today and then they can start their second round of mass arrest," said a protester online.

On Saturday 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 on Friday said three lawmakers -- and a district councillor -- had also 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 with 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 on Friday reported that fresh military anti-riot drills were held across the border in Shenzhen.