Several prominent Hong Kong democracy activists have been arrested in less than 24 hours amid a police crackdown.

Joshua Wong and Agnes Chow of the political party Demosisto were detained on Friday over a 21 June rally where protesters blockaded police headquarters for 15 hours.

Independence campaigner Andy Chan was arrested at the airport on Thursday while trying to fly to Japan.

They are among 900 people arrested since protests began in June.

Lawmaker Cheng Chung-tai of the localist Civic Passion party was also detained, his office said. It was unclear why he was held.

Demonstrations against a now-suspended extradition bill have since turned into wider demands for more autonomy in the former British colony.

Hong Kong police on Friday appealed to members of the public to cut ties with “violent protesters”, and warned people not to take part in a march originally planned for Saturday which had not received official approval.

Why were they arrested?

The Demosisto party said Mr Wong, 23, was “suddenly pushed into a private car on the street” while walking to a train station at around 07:30 local time (23:30 GMT Thursday).

Both he and Ms Chow have been taken to police headquarters in Wan Chai.

Both activists were detained on suspicion of “inciting others to participate in an unauthorised assembly” and “knowingly taking part in an unauthorised assembly”.

They are accused of joining illegal protests and encouraging others to do so.

Mr Wong faces a further charge of “organising an unauthorised assembly”.

Andy Chan, founder of the Hong Kong National Party which campaigns for the territory’s independence, said he was detained on Thursday night while trying to board a flight from Hong Kong airport.

He was arrested on suspicion of rioting and assaulting a police officer, according to local outlet HKFP.

Who is Joshua Wong?

Joshua Wong is a well-known pro-democracy activist who played a leading role in the 2014 rallies known as Hong Kong’s “Umbrella protests” – so-called because protesters used umbrellas to shield themselves from police pepper spray.

Thousands joined marches demanding the right for Hong Kong to choose its own leader – and student leader Mr Wong became the movement’s poster boy.

His latest arrest comes just weeks after he was released from prison on 17 June.

The recent protests have been characterised as leaderless – and activist Nathan Law, who co-founded Mr Wong’s party Demosisto, said nobody was inciting protesters.

“There is no leader or platform in this movement,” he said in a statement. “If someone is inciting citizens to go to the streets, it must be the harsh political violence of [Hong Kong’s leader] Carrie Lam.

“Demosisto has never been ‘leaders’ of the movement. Every Hong Kong citizen who has come out has done so according to his own conscience. No matter how the Chinese Communist Party attempts to smear this, nothing can change that fact.

“We appeal to the public not to be afraid of political violence and white terror and continue to fight for their rights. Hong Kong people, go!”

Are further protests planned?

A protest organised by the Civil Human Rights Front – which has assembled several mass rallies – had been planned for this Saturday but has now been cancelled.

Police had declined permission for the rally, citing public safety concerns. It would have marked the 13th consecutive weekend of protests had it gone ahead.

Bonnie Leung, vice-convener of the CHRF, said the decision followed Mr Wong and Ms Chow’s arrests.

“I think the police are using all kinds of excuses to arrest all kinds of people, including us,” she told the Guardian.

“They arrested Joshua Wong and Agnes Chow this morning so there is a real danger we could face the same consequences as well.”

The convenor of the CHRF, Jimmy Sham, said he had been attacked on Thursday by two masked men who were wielding a baseball bat.

He said he had not been hurt in the encounter, though a friend who shielded him from the attack suffered injuries.

Reasons and background of protests in HK?

The protests began as rallies against a controversial extradition bill – now suspended – which would have allowed criminal suspects to be sent to mainland China for trial.

They have since expanded in scope, becoming a broader pro-democracy movement.

Beijing has repeatedly condemned the protesters and described their actions as “close to terrorism”. Reuters news agency on Friday reported that earlier in the summer China denied a request by Carrie Lam to fully withdraw the extradition bill to help ease tensions and end the unrest.

The protests have frequently escalated into violence between police and activists, with injuries on both sides, and activists are increasingly concerned that China might use military force to intervene.

On Thursday, Beijing moved a new batch of troops into Hong Kong. Chinese state media described it as a routine annual rotation.

But on Friday, an editorial in the China Daily newspaper emphasised that the presence of Chinese troops is not symbolic, and they will have “no reason to sit on their hands” if the situation deteriorates.