Prominent Hong Kong pro-democracy activist Joshua Wong has been arrested in Hong Kong, according to a statement by the political party he co-founded.

The Demosisto party said he had been “forcefully pushed into a private minivan on the street” while walking to a train station at around 07:30 local time (23:30 GMT Thursday).

The 23-year-old was taken to police HQ in Wan Chai, his party said.

Another prominent activist, Agnes Chow, was also arrested this morning.

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

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

Both their arrests are related to a protest on 21 June which saw protesters blockade police headquarters for 15 hours.

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• Joshua Wong, the poster boy for Hong Kong protests

The detentions come a day ahead of a planned demonstration scheduled for this weekend – which would be the 13th weekend in a row that protests have occurred. Police have declined permission for the rally, citing public safety concerns – but protesters are expected to gather regardless.

Well-known independence campaigner Andy Chan said he too had been detained by police, while trying to board a flight from Hong Kong to Japan on Thursday night.

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

Some 900 people have been arrested since protests began in early June, triggered by a controversial extradition bill, police said on Wednesday.

With the bill now suspended, the rallies have evolved into a broader movement demanding democratic reform and an investigation into alleged police brutality.

The protests have frequently escalated into violence between police and activists, with injuries on both sides.

There are also growing fears that China could intervene in the protests by sending in mainland troops. On Thursday, these concerns were fuelled after China’s military moved a new batch of troops into Hong Kong. Chinese state media described it as a routine annual rotation.

The recent protests have been characterised as leaderless, but Joshua Wong is a figurehead for many due to his leading role in the 2014 rallies known as Hong Kong’s “Umbrella protests”.

His latest arrest comes just weeks after he was released from prison in June, after serving time for his role in the 2014 protests.

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It was in 2014 that the activist became the poster boy for the city’s resistance, as thousands marched demanding the right for Hong Kong to choose its own leader.

The Umbrella protests were so-called as protesters frequently used umbrellas to shield themselves from pepper spray fired by police.

At the heart of both the Umbrella movement and the current protests is concern about what is perceived to be China’s increasing influence in Hong Kong.

Beijing is highly sensitive about the territory – which is part of China under a “one country, two systems” model.

Hong Kong has its own legal system and borders, and rights including freedom of assembly and free speech are protected.

A guide to the Hong Kong protests

• Summary of the protests in 100 and 500 words
• All the context you need on the protests
• The background to the protests in video
• More on Hong Kong’s history
• Profile of Hong Kong leader Carrie Lam