Prominent democracy activists were arrested Friday in a dragnet across Hong Kong -- a move described by rights groups as a well-worn tactic deployed by China to suffocate dissent ahead of key political events.

The sweep comes after a major rally planned by a civil rights group on Saturday was banned by police on security grounds.

Hong Kong has been locked in three months of political crisis, with increasingly violent clashes between police and protesters that have prompted an escalating public relations campaign from China.

Protesters had planned yet another mass rally on Saturday -- the fifth anniversary of Beijing's rejection of a call for universal suffrage in the semi-autonomous city.

It was a pivotal moment, sparking the 79-day Umbrella Movement in 2014, which seeded the ground for today's protests.

But organisers said Friday afternoon they would not march, complying with the police ban.

Bonnie Leung of the Civil Human Rights Front (CHRF), the avowedly peaceful rally organiser, said it had "no option but to cancel the march tomorrow" after an appeal to hold the rally was rejected.

Early Friday, two of the Umbrella Movement's leaders, Joshua Wong and Agnes Chow -- both still well-regarded among the city's youth -- were arrested in dawn swoops, accused of "inciting others to take part in unauthorised assembly" among other charges.

The pair were charged Friday afternoon and bailed by the court. The main charge carries up to five years in jail.

The arrests are a sign of the "spread of 'white terror' towards Hong Kong protesters", said Issac Cheng of Demosisto, the party co-founded by Wong, deploying a commonly-used term for China's efforts to fragment and harass the protest movement.

Hours earlier, another vocal independence campaigner Andy Chan was detained at Hong Kong's airport.

A fourth pro-democracy campaigner, Rick Hui -- a district councillor -- and former student leader Althea Suen were also arrested separately.

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

But that has failed to snuff out a leaderless movement, which says freedoms in the city, unique within China, are being eviscerated by Beijing.

Amnesty International decried Friday's "ludicrous dawn swoops", condemning the arrest of Wong and Chow as an "outrageous assault on the rights to freedom of expression and peaceful assembly".

The arrests are "scare tactics straight out of Beijing's playbook", it added.

Police denied the slew of arrests were made specifically to defang the weekend's protests.

"The allegation that we time our arrests is totally false," spokesperson John Tse told reporters.

- Defy the ban? -
Hong Kong's crisis-hit government is scrambling to find an appropriate response to the unprecedented pro-democracy protests, which have by turns seen millions march, closed the airport and left city streets strewn with bricks and shrouded in tear gas.

The protests started as a pushback against a bill allowing extraditions to mainland China, but quickly morphed into wider calls for democracy and police accountability.

While permission for another mass rally on Saturday was denied on security grounds, pockets of protesters swiftly vowed to hold creative events at the scheduled time and place to sidestep the ban.

Those included a mass shopping trip, football match and impromptu religious gatherings in downtown Hong Kong, while a YouTuber with 800,000 followers called a fan meeting.

With a hardcore minority among the protesters, mainly young students, unlikely to heed the police ban, the weekend could be poised for renewed violent clashes.

Student protester Kelly, who wanted to be identified only by her first name, said the arrests would not cow the movement.

"The police think there are leaders behind the protests and this will stop us," she said.

"We are our own leaders and we will keep coming out."

On Sunday, the city saw some of its worst clashes, with running battles between black-clad protesters, armed with bricks and Molotov cocktails, and police wielding batons, rubber bullets and tear gas.

One officer fired a warning shot -- believed to be the first live round used during the protests -- as a mob with sticks set upon several policemen.

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

China has responded with a campaign of intimidation, with a slick PR video released Thursday showing troop movements into Hong Kong as part of "routine garrison rotation".

State-media reported that fresh Chinese military anti-riot drills were held across the border in Shenzhen.

Businesses in the financial hub -- from airline Cathay Pacific to the city's metro operator -- have also been squeezed by Beijing for harbouring apparent supporters of the pro-democracy movement.

Cathay sacked several staff over the protests after China's aviation regulator banned pro-democracy supporters among the carrier's staff from its airports and airspace.

On Friday, the airline went further, threatening to sack any staff who join a scheduled two-day strike next week.