Three of Hong Kong’s most prominent pro-democracy activists have been arrested ahead of another weekend of planned protests in the Chinese-ruled city.

Leading activist Joshua Wong, 22, was arrested while walking to a subway station about 7.30am on Friday, according to a statement from his political organisation, Demosisto.

The group said Mr Wong was “forcefully pushed into a private minivan on the street” and taken to police headquarters in Wan Chai.

Demosisto co-founder Nathan Law said in a media WhatsApp group that another party member, Angus Chow, had been arrested at her home. She was also being taken to police headquarters.

Hong Kong police said Mr Wong and Ms Chow were being investigated for their roles in a June 21 unauthorised protest outside a police station.

Both face potential charges of participating in the demonstration and inciting others to join it. Wong is also being investigated on suspicion of organising it.

BREAKING: Our secretary-general @joshuawongcf was just arrested this morning at roughly 7:30, when he was walking to the South Horizons MTR station. He was forcefully pushed into a private minivan on the street in broad daylight. Our lawyers following the case now.

— Demosistō 香港眾志 😷 (@demosisto) August 30, 2019

Earlier, the leader of the banned pro-independence Hong Kong National Party, Andy Chan, said he was detained on Thursday night while trying to board a flight to Japan.

The Hong Kong Free Press reports that Mr Chan said in a Facebook message that his detention was requested by police. A police spokesperson told HKFP that Mr Chan was arrested on suspicion of rioting and assaulting a police officer.

Joshua Wong and Agnes Chow at protests in June. Photo: AAP

The arrests came on the day that the Beijing-run English-language newspaper China Daily warned that Chinese troops stationed in Hong Kong were not there for symbolic purposes and would have “no reason to sit on their hands” if violence in the city worsened.

China on Thursday completed what it called a routine rotation of the air, land and maritime forces stationed in Hong Kong, which has been rocked since June by a wave of sometimes violent demonstrations.

State-run media released pictures of armed personnel carriers and trucks carrying troops into Hong Kong before dawn on Thursday.

“Notwithstanding its routine nature, the move touched the nerve of some people in Hong Kong, who have jumped to the conclusion that it is associated with the current dissension in the city,” the newspaper said in an editorial in Friday.

Chinese military vehicles on their way into Hong Kong on Thursday. Photo: Xinhua News Agency

A detachment of People’s Liberation Army soldiers has been stationed in Hong Kong since the 1997 handover that ended British rule. They stage frequent drills but are seldom seen outside their bases.

Analysts estimate China has 8000 to 10,000 troops split between Hong Kong and bases in southern China.

The possibility of their deployment to quell the protests has hung over Hong Kong. The city’s government, which ultimately answers to Beijing, has repeatedly said it can handle the situation.

“While the SAR government has so far not felt the need to call on the garrison, that does not mean it will not do so should the situation demand it,” the China Daily editorial said.

“If the already ugly situation worsens, with the violence and unrest threatening to spiral out of control under the orchestration of secessionist-minded troublemakers, the armed forces stationed in the SAR will have no reason to sit on their hands.

“The PLA garrison in Hong Kong is not merely a symbol of Chinese sovereignty over the city.”

Hong Kong’s mini-constitution, the Basic Law, states that the former colony can request the garrison’s assistance to maintain public order but “they shall not interfere in local affairs”.

Unrest in Hong Kong escalated in mid-June over a now-suspended extradition bill that would have allowed people to be sent to mainland China for trial in Communist Party-controlled courts.

It has since evolved into calls for greater democracy under the “one country, two systems” formula, which guarantees freedoms not enjoyed on the mainland, including an independent judiciary.

On Thursday, Hong Kong police banned a mass protest planned for Saturday that had been called by the Civil Human Rights Front, a key organiser of some of the summer’s recent demonstrations.

-with AAP