Chaotic scenes erupted when several plain-clothes officers pepper-sprayed and wrestled protesters to the ground. Baton-wielding officers then went for other demonstrators and had water bottles thrown at them.

Among anti-government slogans chanted near Jordan MTR station were calls for the city to be independent from Chinese rule.

As of 9pm, police said, one woman had been arrested on suspicion of violating the national security law for chanting slogans that promoted Hong Kong independence.

Some 270 others were detained on suspicion of illegal assembly. Five men were held for disorderly conduct in a public place, another five people for not being able to present their identity cards, and others for assaulting police, obstructing officers in the execution of their duties, loitering or other offences.

A bus driver who reportedly honked his horn at officers blocking a street was also arrested although the force later said he was held on suspicion of dangerous driving as he was going too fast and drove his vehicle very close to officers on Nathan Road, endangering their safety.

Twenty-two people were fined for violating coronavirus-related social-distancing rules that limit public gatherings to two people.

League of Social Democrats activists “Long Hair” Leung Kwok-hung, Raphael Wong Ho-ming and Figo Chan Ho-wun were among those arrested after raising a banner outside the Eaton Hotel in Jordan to criticise the government for postponing the polls.

“I want my right to vote! Shame [on the government] for postponing the elections!” Leung said.

Police accused them of taking part in an unauthorised assembly, and then pointed to the 30 or so journalists following them and said they also could be considered as taking part in the illegal event.

Officers later took the trio into a police vehicle. A post on Chan’s Facebook page subsequently said the three had been arrested.

Hongkongers were supposed to be casting their votes on Sunday to choose lawmakers for the next term. But the government postponed the elections in late July, citing public health risks because of the resurgent coronavirus outbreak.

Critics, however, said the move was politically motivated. Opposition politicians had been banking on winning an unprecedented majority in the 70-seat legislature, buoyed by their trouncing of pro-establishment rivals in last year’s district council polls.

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A 63-year-old protester said she was not worried despite police being out in force. The woman, named Yim, was unhappy about the postponement of the elections.

“Why not let me vote? You let people do mass virus testing, but why not let people participate in an election?” she said, referring to the government’s citywide voluntary coronavirus screening scheme. “If you are postponing the elections for one month, that would still be acceptable. But that’s not the case now. Why does it have to be so?”

A protester in her 20s said: “I feel more pressure to come out [and protest]. We never know when police will arrest us with the national security law. But Hongkongers have no more room to retreat.”

A 20-year-old said he was afraid of being arrested but it was his responsibility to come out and defend citizens’ freedom of speech.

At around 2.15pm, activist Joshua Wong Chi-fung appeared outside the Eaton Hotel. Asked whether he was there to join the protest, Wong said he would “see what happens”, because of the number of police officers present.

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But he said there were four demands which he wished to express, including withdrawing the national security law and relaunching the elections immediately.

“We hope the world can never forget September 6 should be election day. And now Beijing delayed and even cancelled the elections, which is totally unreasonable,” he said.

Dozens of people also gathered inside Langham Place shopping centre in Mong Kok to chant protest slogans. Officers did not enter the mall but checked the identity cards of those outside and nearby.

Small groups of protesters in Mong Kok were still being searched as night fell and, on one occasion, officers would not let people inside a mobile phone shop leave until after their identities were checked.

In a statement in the evening, the government strongly condemned the protest and said such unauthorised assemblies could significantly increase the risk for Covid-19 to spread.

A spokesman said it was international practice for countries to pass laws to protect national safety. The national security law was constitutional, reasonable, and in line with the “one country, two systems” principle, he said, referring to the framework under which Beijing governed Hong Kong.

The law bans acts of secession, subversion, terrorism and collusion with foreign forces.

The spokesman also defended the decision to postpone the elections, saying the Covid-19 outbreak could last for some time and there could be another outbreak during the winter.

Beijing’s liaison office in the city also said in a statement it supported police taking decisive enforcement action to ensure stability.

The office said it was reasonable for the government to postpone the elections, hitting out at opposition figures for inciting “street resistance”. It also criticised these figures for politicising Beijing’s help in Hong Kong’s fight against the coronavirus outbreak, saying they had no conscience.

Police last made arrests on Sunday’s scale on July 1 when 370 people were detained for various offences as thousands gathered in anger over the security law, which had been imposed on the city the previous day.

In a statement on a Telegram group earlier, the anonymous organisers of Sunday’s protest said citizens had to take to the streets because there was already no way out in the face of a “shameless” government.

The protest was also organised to show disapproval about the plans to introduce a health code system, which would allow people to travel freely without need for quarantine between Hong Kong, mainland China and Macau.

Meanwhile, a 12-year-old girl was seen in a viral video being pinned down by riot police on Sai Yeung Choi Street South in Mong Kok at about 4pm. She had tried to run away but was quickly stopped and grabbed by a male officer who then pinned her down to the ground with his knee while trying to subdue her. She and her brother were later fined for violating social-distancing rules.

The siblings sustained minor bruises and were treated at Kwong Wah Hospital in Yau Ma Tei.

Her mother told the media the girl was just going to buy paint with her brother but they were stopped by police. She said she would not pay the fine – a fixed penalty of HK$2,000 (US$256) for each – and planned to file a complaint.

Police said on their Facebook page about 100 people had gathered on the street and were yelling, with some refusing to leave after a warning. Officers intercepted them, including the girl, who “ran away in a suspicious manner”. Officers gave chase and applied minimum force to subdue her, the force said.