Human rights advocates and critics have warned of what they called “increasingly aggressive” policing tactics in Hong Kong after a male officer tackled a 12-year-old girl and pinned her to the ground during Sunday’s anti-government protests.

Although the young girl was not among nearly 300 people arrested that day, images of her being restrained on the streets of Mong Kok angered advocates for women’s and children’s rights, coming just days after officers knocked over a pregnant woman blocks away before using pepper spray.

During the rally in Kowloon on Sunday, police unleashed rounds of pepper balls and pepper spray. Plain-clothes officers wrestled demonstrators to the ground, while water bottles were thrown at police wielding batons who went after other protesters. The response has raised concerns officers were using excessive force at a disturbance much smaller than what regularly occurred during last year’s social unrest.

A police insider said the force used a pre-emptive approach by making early arrests, preventing the demonstration from plunging into further violence.

But an expert struck a more cautious tone, saying the move risked innocent people being detained, and did little to rebuild the police’s image, battered by accusations they used excessive force during the months-long anti-government movement last year.

Sunday’s demonstrators were answering anonymous online calls to protest the Beijing-enacted national security law and the delay of the Legislative Council elections, which the government postponed for a year, citing health risks during the coronavirus pandemic.

Activists could be heard chanting “Liberate Hong Kong; revolution of our times”, a slogan that could run afoul of the new law. Police estimate about 1,000 people took part in the illegal protest, and by day’s end, 289 arrests were made.

The scene involving the 12-year-old girl was caught on video and circulated online. It shows her trying to run from police on Sai Yeung Choi Street South, but the officer grabs her and pins her to the ground with his knee. She was later fined for violating social-distancing rules, along with her brother.

"They could have just stopped her or used other means instead of tackling her to the ground Billy Wong, executive secretary, Committee on Children’s Rights"

In a statement released later, police said she was acting “suspicious”. The force insider said the girl and her brother were briefly held, along with six others, after repeated warnings to disperse. But the girl said during press interviews she was merely there to buy painting tools.

Billy Wong Wai-yuk, executive secretary of the Hong Kong Committee on Children’s Rights, called the officer’s actions “unacceptable”, saying the video footage showed the girl was a minor and unarmed.

“They could have just stopped her or used other means instead of tackling her to the ground,” Wong said.

Wong Chi-yuen, a community officer with the Society for Community Organisation, said such an experience could harm a young person both mentally and physically. Police should have exercised greater judgment to determine whether the girl was a protester before taking action, he said.

During another rally on August 31, a pregnant woman was dragged to the ground on Argyle Street by a group of officers and hit by pepper spray being used near her. Police later said officers were unaware she was pregnant, and they were deeply concerned about the incident.

Mabel Au, chairwoman of the Association for the Advancement of Feminism, said officers seemed to lack the ability to clearly identify members of crowds at protests before taking action against individuals.

Another force insider said officers adopted a strategy of intervening more quickly and taking resolute action to stop large crowds from forming on Sunday, insisting the mass arrest was “appropriate” and “necessary”, as well as in accordance with prima facie evidence.

The number of arrests at the weekend was the second-highest since police adopted the new strategy earlier this year. More than 370 people were arrested on July 1 during the annual protest on the anniversary of the city’s return to Chinese rule. Authorities cited health risks in banning the event.

“Taking firm and decisive action at the outset can prevent more serious crimes and send a clear message to others taking part or thinking of taking part,” said another police source, referring to scenes of vandalism and arson during last year’s anti-government unrest.

Former director of public prosecutions Grenville Cross said that if police had reasonable suspicion people were taking part in an unlawful assembly, officers had to act. “They could not simply turn a blind eye to what they had done, and they would have had sufficient legal grounds for arresting them,” he said.

Lawrence Ho Ka-ki, a scholar who specialises in policing at Education University, said the mass arrest strategy served as a successful deterrent to stop protesters from gathering, helping the force regain control. But for members of the public who already harboured doubts over police behaviour, the approach would only increase their mistrust.

The foreign affairs spokeswoman for the European Union, Nabila Massrali, also raised concerns over freedom of speech in Hong Kong following the detentions.

“The arrests of hundreds of individuals for engaging in peaceful protests raises serious concerns about the protection of fundamental freedoms in Hong Kong,” Massrali said. “It is essential that the rights and freedoms of Hong Kong residents are fully protected, including freedom of speech and of the press, as well as freedom of association and of assembly.”

Among those arrested on Sunday was a bus driver who reportedly honked his horn at officers blocking a street. The force later said he was held on suspicion of dangerous driving, and on Monday accused him of also possessing an offensive weapon – a spanner. A staff union for New World Bus drivers said it was normal for drivers to carry the tool to adjust their rear-view mirrors.