Thousands of Chinese troops streamed into Hong Kong in the small hours of Thursday morning, prompting alarmed witness reports after weeks of military intervention threats from Beijing amidst the city’s protest crisis.

Chinese state media later described the move as an “annual… routine” rotation of Hong Kong’s permanent garrison.

But the scenes of armoured carriers and trucks moving in convoy through the city streets, and the manner of the announcement in the official Xinhua news agency, led pro-democracy activists to call it yet another “warning” that China is willing to use force to bring the ongoing unrest to an end.

It is more than 80 days since protests began over a now-suspended extradition bill that would have made it easier for Hong Kong to deport criminal suspects to the mainland. The protests have spiralled into the biggest broad-based political challenge to Beijing’s rule since the city was handed back to China by Britain in 1997.

China has made clear in recent weeks that it considers a military intervention in the crisis a viable option, despite US warnings that it would risk a repeat of the 1989 Tiananmen Square massacre.

Xinhua quoted the People’s Liberation Army (PLA) as saying that a handover ceremony for the Hong Kong garrison was held overnight.

It quoted an army major as telling the newly stationed troops: “This time the task has a glorious mission. The responsibility is great. The job is difficult. The time for a true test has arrived.”

Xinhua said the soldiers in the new rotation were educated on Hong Kong’s laws and vowed to defend the nation’s sovereignty.

Liu Zhaohui, the garrison’s deputy chief of staff, said on the CCTV channel that the PLA would “firmly implement the guideline of ‘one country, two systems’” – the principle by which Hong Kong is considered a part of China but with an independent judiciary and certain democratic rights not enjoyed by those on the mainland.

Experts noted that, while the garrison change had indeed been expected, reports in Chinese media on the previous troop rotations in 2017 and 2018 made no reference to defending “one country, two systems”.

These previous notices also specified that the number of soldiers and amount of equipment in the Hong Kong garrison “was maintained with no change”. There was no such statement in Thursday’s announcement.

Dennis Kwok, an MP with the pro-democracy Civic party, questioned the timing and prominent Chinese media coverage of the troop movements.

“I believe this is a deliberate posture on the part of the PLA to tell or warn the Hong Kong people that it may be deployed,” he told public broadcaster RTHK.

“As I said time and again, the use of troops in Hong Kong will be the end of Hong Kong, and I would warn against any such move on the part of the central people’s government.”

Protest leaders are planning another major march for this coming Saturday, marking five years since Beijing announced it would pre-approve all candidates running for chief executive of the city in elections – effectively denying the people of Hong Kong universal suffrage.

Continuing protests rock Hong Kong: In pictures Show all 35 1 /35 Continuing protests rock Hong Kong: In pictures Continuing protests rock Hong Kong: In pictures Police officers point their guns towards pro-democracy protesters after a clash at a march in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures Pro-democracy protesters throw molotov cocktails during clashes with riot police in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures Demonstrators rally behind barricades ass they are shrouded in tear gas deployed by police in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures Police clash withpro-democracy protesters at a march in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures A police officer prepares to strike a protester as clashes erupt during a pro-democracy march in Hong Kong on August 24 AFP/Getty Continuing protests rock Hong Kong: In pictures A water cannon is fired by riot police to clear the street of demonstrators during a protest in Hong Kong on August 25 EPA Continuing protests rock Hong Kong: In pictures Pro-democracy protesters use a slingshot during clashes with riot police in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures Riot police fire tear gas to disperse pro-democracy protesters in Hong Kong on August 24 Reuters Continuing protests rock Hong Kong: In pictures A demonstrator throws a molotov cocktail as they clash with riot police during a protest in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures A demonstrator swings a tennis racket in an attempt to knock back a tear gas cannister thrown by riot police during a protest in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures A protester displays a Hong Kong Independence flag during a pro-democracy march in Hong Kong on August 24 EPA Continuing protests rock Hong Kong: In pictures Riot police tend to a detained demonstrator who has been injured during a protest in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures A demonstrator uses bamboo poles to build a barricade during a protest in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures Riot police clash with demonstrators during a protest in Hong Kong on August 24 Reuters Continuing protests rock Hong Kong: In pictures Protesters are hit by tear gas fired by the police in Hong Kong on August 24 AFP/Getty Continuing protests rock Hong Kong: In pictures Demonstrators shrouded in tear gas shield themselves behind barricades during a clash with riot police in Hong Kong on August 25 Getty Continuing protests rock Hong Kong: In pictures Riot police arrest a protester during a pro-democracy march in Hong Kong on August 24 EPA Continuing protests rock Hong Kong: In pictures Police clash with pro-democracy protesters at the Hong Kong international airport on August 13 AFP/Getty Continuing protests rock Hong Kong: In pictures Protesters fall back as they clash with police during a protest at Kowloon Bay in Hong Kong on August 24 AFP/Getty Continuing protests rock Hong Kong: In pictures Police clash with pro-democracy protesters at the Hong Kong international airport on August 13 Reuters Continuing protests rock Hong Kong: In pictures Riot police move to disperse protesters during a clash at the Hong Kong international airport on August 13 Reuters Continuing protests rock Hong Kong: In pictures Commuters riding in a bus drive past protesters in Hong Kong's Kowloon Bay on August 24 AFP/Getty Continuing protests rock Hong Kong: In pictures Protester hold umbrellas as they take part in a pro-democracy march in Hong Kong on August 24 EPA Continuing protests rock Hong Kong: In pictures Riot police detain a demonstrator as they clash during a protest in Hong Kong on August 24 Reuters Continuing protests rock Hong Kong: In pictures Pro-democracy protesters block the entrance to the airport terminals after clashes with police at the Hong Kong international airport on August 13 AFP/Getty Continuing protests rock Hong Kong: In pictures Riot police spray pepper spray as shield themselves by a police van during a clash with pro-democracy protesters at the Hong Kong international airport on August 13 Reuters Continuing protests rock Hong Kong: In pictures Riot police use pepper spray to disperse protesters during a demonstration at the Hong Kong international airport on August 13 Reuters Continuing protests rock Hong Kong: In pictures Demonstrators shrouded in tear gas shield themselves behind barricades during a clash with riot police in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures Medical professionals gather to protest police brutality at the Queen Elizabeth Hospital in Hong Kong on August 13 Getty Images Continuing protests rock Hong Kong: In pictures Police detain a pro-democracy protester at the Hong Kong international airport on August 13 AFP/Getty Continuing protests rock Hong Kong: In pictures A protester is halted by police during an occupation at the Hong Kong international airport on August 13 AP Continuing protests rock Hong Kong: In pictures Police clash with pro-democracy protesters at the Hong Kong international airport on August 13 AFP/Getty Continuing protests rock Hong Kong: In pictures Police stand guard at the entrance of the departure terminals at the Hong Kong international airport on August 13 AFP/Getty Continuing protests rock Hong Kong: In pictures Protesters continue their occupation at the Hong Kong international airport on August 14 AP Continuing protests rock Hong Kong: In pictures Travellers read placards and posters left by pro-democracy protesters at the Hong Kong international airport on August 14 AP

Hundreds of thousands are expected to attend, despite police writing to organisers Civil Human Rights Front (CHRF) on Thursday refusing permission for the march. The group said it will appeal the decision.

On Wednesday night, protesters gathered to express anger at a specific issue they have with the conduct of the city’s police in recent weeks, raising allegations of what protest leader Joshua Wong called “sexual violence” towards female protesters.

The rally was billed as a #MeToo event, part of the global movement to raise awareness of sexual assault and harassment in diverse industries including cinema, politics and the media.

Women at the rally gave accounts of mistreatment by police officers during pro-democracy protests. One woman broke down in tears as she described being strip-searched, according to the South China Morning Post.

Another said officers had called her a “prostitute” as they dragged her away from a protest, with her dress upturned.