Prominent democracy activists have been arrested across Hong Kong - a move described by rights groups as a well-worn tactic deployed by Beijing to dampen dissent ahead of key political moments.

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

Two Hong Kong pro-democracy lawmakers have been arrested late today, the Civic Party said, bringing to three the number of members of the city's parliament targeted in a protest crackdown.

Au Nok-hin and Jeremy Tam were arrested on suspicion of obstructing police, while Au was also accused of assaulting a police officer, Tam's Civic Party said on its verified Facebook page.

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.

Jeremy Tam (R) with a group of Hong Kong protesters pictured in July

Protesters had planned yet another mass rally tomorrow -  the fifth anniversary of Beijing's rejection of a call for universal suffrage in the semi-autonomous city, a decision that sparked the 79-day Umbrella Movement in 2014.

But organisers said they would not march, complying with the police banning order.

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

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

Hours before another vocal independence campaigner was detained at Hong Kong's airport.

Agnes Chow leaves the court after being arrested and released on bail

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

More than 850 people have been arrested in connection with protests since June, including prominent independence campaigner Andy Chan who police detained at Hong Kong airport last night on suspicion of rioting - a charge which carries up to ten years in prison.

Chan's small independence party was outlawed last year on the grounds it posed a national security threat, the first such ban since the former British colony returned to Chinese rule in 1997.

Chinese state media described the three arrested as "independence" advocates.

A fourth pro-democracy campaigner, Rick Hui, a counsellor for the working-class Sha Tin district, was also detained by police, according to a post on his Facebook page, without detailing why.

Amnesty International decried the "the ludicrous dawn swoops", condemning the arrest of Mr Wong and Ms Chow as an "outrageous assault on the rights to freedom of expression and peaceful assembly" and as "scare tactics straight out of Beijing's playbook".

Joshua Wong pictured during a rally in July

The arrests come as Hong Kong's crisis-hit government scrambles 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 kickback against a bill allowing extraditions to mainland China, but quickly billowed out into wider calls for democracy and police accountability.

Permission for another mass rally tomorrow was denied on security grounds, raising the likelihood of another weekend of clashes between police and protesters.

In a letter to rally organisers - the Civil Human Rights Front (CHRF) - police said they feared some participants would commit "violent and destructive acts".

After an appeal to hold the rally was rejected, Bonnie Leung of the avowedly peaceful CHRF said "no option but to cancel the march tomorrow".

But pockets of protesters swiftly vowed to hold creative events at the scheduled time and place of the rally, including 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 appeared poised for renewed violent clashes.

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, during which troops used water cannon to disperse crowds throwing petrol bombs.

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 body banned pro-democracy supporters among the carrier's staff from its airports and airspace.

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