HONG KONG - Thousands are gathering for rallies in various sites across Hong Kong on Monday (Sept 2) as anti-government protesters have called for a city-wide strike and a boycott of classes on what is meant to be the first day of the new school term while government officials say they are not ruling out imposing emergency law.

At Tamar Park adjacent to the government headquarters, tens of thousands gathered under an overcast sky, with young and old covering the lawn and tiled walkways, listening to organisers and guests speak about their thoughts on the anti-extradition movement.

Ms Maggie Chung, 32, told The Straits Times she took sick leave to join the strike, adding that she has been helping out at the frontlines of clashes with the police by offering saline to protesters affected by tear gas since June 9.

Asked how long she can continue, the accountant said: "I'm actually very afraid but someone has to go out there to press the government so I will join for as long as I can."

As for the trashing and vandalism of train stations and other public properties, Ms Chung said the root of the actions stem from the government's inaction and non-responses.

Another rally participant, Mr Edwin Chan, 32, told The Straits Times he came after work as this is one of the things he can do. Unlike most rally participants, the employee at a luxury watch retailer did not cover his face with a mask and was dressed in office wear.

Asked about the inconveniences caused by the disruption campaigns to prevent trains from leaving and the destruction of public and train station facilities, Mr Chan said the hardcore protesters will have to face the rule of law eventually.

"The problem here is that the police have also breached the law by using excessive force but the government is not taking them to task so it's very unfair.

"What some police have done when making arrests is to pin protesters to the ground and beat them up with batons till they bleed. You can arrest them, but they already cannot resist so why beat them up?"

After the rally, some protesters gathered outside the People's Liberation Army garrison nearby where they were kept at bay by police with pepper spray.

The crowd mostly dispersed by 8pm but regrouped about an hour later outside the Mong Kok Police Station in Prince Edward, where they shone laser pointers at the building and chanted "Hong Kong Police, know the law and break the law".

When groups of officers emerged in riot gear and at least two police dogs, the crowd jeered and shouted "leave!".

Earlier in the morning, groups of black-clad protesters with face masks started a planned disruption of the city's subway services on a rainy Monday morning (Sept 2), in what is the 13th straight week of escalated protests, ahead of a planned two-day strike starting that day.

Adopting the mantra "be like water" to adapt to changes in circumstances, they gathered from 7.20am at various MTR stations including Kowloon Tong, Fortress Hill, Yau Ma Tei, Prince Edward and Mong Kok, with police officers on standby in various places to counter them.

To prevent trains from leaving the stations, protesters sandwiched tissue packets in between train doors or blocked commuters with umbrellas or their bodies.

Meanwhile, Hong Kong's No. 2 official Matthew Cheung would not rule out imposing an emergency law to help police contain protests that have led to increased violence since they began in June.

"We are reviewing comprehensively with an open attitude on what could be used," Mr Cheung told reporters on Monday when asked whether the government would use an emergency law, adding that any actions would need to be "reasonable".

He was repeating comments made last week by Mrs Lam about the sweeping colonial-era law, which allows for easier arrests, deportations, censorship and property seizures. It has only been invoked once - during riots in 1967.

"Once calm's restored, society's back to normal, then we can go forward," Mr Cheung said. "Law and order must be restored ASAP, without further ado. No nonsense. We are all yearning for law and order."

Railway operator MTR has been a target of protesters, particularly after it was granted a court injunction to shut stations on protest days. Chinese state media had previously publicly criticised MTR for helping protesters get away after clashes with the police.

On Monday, the railway operator said all stations would open even though 12 stations were damaged on Sunday and 32 last Saturday. Some protesters had trashed Tung Chung station on Sunday, ripping apart the station turnstiles and breaking ticket machines.

This came after they choked travel routes to the airport by setting up barricades at Sky City Interchange and Chek Lap Kok South Road. They used water-filled barriers, rubbish dumpsters and luggage trolleys, and hurled objects at officers and Airport Authority staff.

Police have warned that the protesters could be in contempt of court, given the indefinite court order banning actions that would obstruct the proper use of the airport and passageways nearby. The protesters' action on Sunday led to 25 flight cancellations, said the Airport Authority.

In a press conference on Monday afternoon, police said 159 people were arrested after the weekend's violence, which included illegal assembly, carrying unauthorised weapons and assaulting police. This brings the total arrested thus far to 1,117.

Condemning the weekend's activities, they also clarified that the protester photographed holding a petrol bomb with what seems to be a pistol strapped to his belt is not an undercover officer, and that the weapon is an imitation firearm and not one issued by the police. Officers noted that a number of imitation weapons, air guns and BB pellets have been spotted at recent protests.

"It is very dangerous to be carrying an imitation weapon during such circumstances because no one can tell the difference in the chaos, and if anyone points such a weapon at the police, officers will have to respond with equal force," said Assistant Commissioner of Police Mak Chin-ho.

"The result may be far beyond anyone's expectations."

Separately, students in high schools marked the start of their new school year by boycotting classes on Monday, when the new school term began.

Some wore gas masks, helmets and goggles - the now essential kit carried by protesters during months of tear gas-enveloped rallies and clashes with police, AFP reported.

At one school, the bronze statue of Sun Yat-sen, the Hong Kong-educated powerhouse of Chinese political thought, was also fitted with a gas mask and goggles.

Outside the Chinese University of Hong Kong, one of the city's largest, hundreds of students took turns to make speeches from a stage with a black backdrop embossed with "Students in Unity Boycott for our City", Reuters reported.

"I come here just to tell others that even after summer holidays end, we are not back to our normal life; we should continue to fight for Hong Kong," said one 19-year old student who asked to be identified as just Chan.

"These protests awaken me to care more about the society and care for the voiceless."

Hong Kong has been roiled by unrest since demonstrations escalated from June 9. What began in late March as opposition to a controversial extradition Bill proposed by Mrs Lam has turned into a wider movement encompassing five demands.

They are: a complete withdrawal of the Bill instead of just an indefinite suspension; an inquiry into alleged use of excessive force by the police; removal of the "riot" label on the June 12 protest; the release of all protesters arrested; and the implementation of universal suffrage.

The Bill would have Hong Kong extradite suspects to various jurisdictions, including mainland China.

But many doubt the Chinese legal system despite reassurances from the Hong Kong government that measures are in place to protect the rights of suspects to receive a fair trial.

Business sentiments are shaken and revenues have fallen, adding on to the city's strains from the United States and China tussle, slowdown in China's economy and weak Chinese yuan.

The months of protests have piled on the uncertainty and taken a toll on tourism and the pivotal retail industry.

Mr Edwin Chan, who sells luxury watches, said now that mainland tourists - who form a bulk of clientele in the luxury retail sector - are avoiding Hong Kong, his sales plummeted 90 per cent in recent weeks in light of the unrest.

"It's hard, but we are doing this to have a future," he said.

And the woes are far from over, with Hong Kong on the brink of its first recession in a decade, with Financial Secretary Paul Chan lowering its full-year 2019 growth forecast to 0-1 per cent, down from 2-3 per cent previously.