Chinese military vehicles have been seen moving across the border into Hong Kong, in what the military said were regular troop movements, as fears rose that the city could see a Beijing-led crackdown after months of political unrest.

Following witness reports of the movements in the early hours of Thursday, state-run news agency Xinhua released a report that the Hong Kong Garrison of the Chinese People’s Liberation Army (PLA) was making a scheduled rotation and that it was an “annual normal routine”.

The images published by Xinhua showed armoured carriers and trucks carrying troops at the border, and a naval vessel arriving in Hong Kong.

In the previous two rotations – in 2017 and 2018 – state media reports noted that the number of troops and equipment had not changed. This year the report does not include that detail. It is estimated there are between 8,000 and 10,000 troops in the garrison, on either side of the border.

The movements come before a major anti-government demonstration planned for Saturday, as Hong Kong nears its third month of mass protests. On Thursday, the organiser of the demonstration, Civil Human Rights Front, received notice that police had banned the event.

Quick Guide What are the Hong Kong protests about? Show Why are people protesting? The protests were triggered by a controversial bill that would have allowed extraditions to mainland China, where the Communist party controls the courts, but have since evolved into a broader pro-democracy movement. Public anger – fuelled by the aggressive tactics used by the police against demonstrators – has collided with years of frustration over worsening inequality and the cost of living in one of the world's most expensive, densely populated cities. The protest movement was given fresh impetus on 21 July when gangs of men attacked protesters and commuters at a mass transit station – while authorities seemingly did little to intervene. Underlying the movement is a push for full democracy in the city, whose leader is chosen by a committee dominated by a pro-Beijing establishment rather than by direct elections.

Protesters have vowed to keep their movement going until their core demands are met, such as the resignation of the city’s leader, Carrie Lam, an independent inquiry into police tactics, an amnesty for those arrested and a permanent withdrawal of the bill. Lam announced on 4 September that she was withdrawing the bill. Why were people so angry about the extradition bill? Beijing’s influence over Hong Kong has grown in recent years, as activists have been jailed and pro-democracy lawmakers disqualified from running or holding office. Independent booksellers have disappeared from the city, before reappearing in mainland China facing charges. Under the terms of the agreement by which the former British colony was returned to Chinese control in 1997, the semi-autonomous region was meant to maintain a “high degree of autonomy” through an independent judiciary, a free press and an open market economy, a framework known as “one country, two systems”. The extradition bill was seen as an attempt to undermine this and to give Beijing the ability to try pro-democracy activists under the judicial system of the mainland. How have the authorities responded? Beijing has issued increasingly shrill condemnations but has left it to the city's semi-autonomous government to deal with the situation. Meanwhile police have violently clashed directly with protesters, repeatedly firing teargas and rubber bullets. Beijing has ramped up its accusations that foreign countries are “fanning the fire” of unrest in the city. China’s top diplomat Yang Jiechi has ordered the US to “immediately stop interfering in Hong Kong affairs in any form”. Lily Kuo and Verna Yu in Hong Kong

Hours after the announcement, Civil Human Rights Front convenor Jimmy Sham and a friend were attacked inside a restaurant by two men in masks wielding baseball bats and knives.

While Sham was unharmed, his friend is receiving treatment for injuries sustained during the attack, according to the coalition.

Bonnie Leung, a co-convenor of the group, said she thought the attack was intended to scare away protesters from Saturday’s demonstration. Past events organised by CHRF this summer have drawn hundreds of thousands of residents, including families and the elderly, in some of the largest demonstrations seen in Hong Kong since it returned to Chinese rule in 1997.

“My guess is, whoever attacked [or] hired people to attack Jimmy want to … scare people away from protests,” she said. “But I don’t think it will work. Hong Kong people knows the importance of unity - the more people come out, the safer everyone will be.”

The attack is not unprecedented in Hong Kong, which has seen an escalation of violence targeted at protesters, journalists and pro-democracy figures in recent weeks.

Last week, three people were stabbed at a “Lennon Wall” in eastern Hong Kong, one of many temporary collections of posters and notes that have gone up across the city to support protesters, according to Hong Kong Free Press.

In separate incidents, groups of stick-wielding men attacked protesters in Yuen Long after a demonstration on July 21 and again at a demonstration in Causeway Bay on August 5.

Hong Kong lawmaker Dennis Kwok of the Civic party called the troop movements political posturing. “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,” Kwok 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.”

A Chinese naval vessel docks in Hong Kong on Thursday. Photograph: Xinhua

The protests, triggered in June by an extradition bill that would send suspects to mainland China, represent the sharpest challenge to Beijing’s authority since the handover. In recent weeks, China has issued increasingly severe warnings.

The PLA released a propaganda video earlier this month showing armed police and soldiers subduing people in yellow hard hats and black tops, the unofficial uniform of the Hong Kong demonstrators.

As concerns about a possible crackdown intensify in Hong Kong, the Hong Kong Internet Service Providers Association said on Wednesday it was troubled by reports that the local government might issue an executive order to block selective websites or applications.

It said such an order would be the “end of the open Internet of Hong Kong” and “permanently deter international businesses from positing their businesses and investments in Hong Kong”.

The group also said it would be an ineffective move as it “would not deter nor stop determined users from accessing their desired services” through a virtual private network (VPN).