More than a million people took to the streets of Hong Kong on New Year's Day in a resounding reminder to the city authorities that the past six months of pro-democracy protests will continue into 2020.

Crowds of people of all ages including families with young children converged on the city's Victoria Park for a peaceful and officially approved march that was nonetheless attacked by riot police wielding tear gas.

"We have to walk side by side without losing sight of our initial aims," Jimmy Sham, convenor of march organizers the Civil Human Rights Front, told the crowd.

"We have seen the government crack down on the Spark Alliance [funding campaign for arrestees], on teachers, and maybe even civil servants will be next."

"This is about all of us, and we won't give up the fight nor forget those who have been targeted for suppression," he said. "We will walk hand-in-hand with them and continue to fight for the five demands."

Singing the protest movement anthem, Glory to Hong Kong, and holding up their hands to signal the five demands of the movement, the crowd moved at glacial speed in the direction of the Central business district, hampered by the sheer numbers trying to join the throng from all directions.

"Don't fear suppression, walk side by side!" the protesters chanted, as well as the now-familiar "Five demands, not one less!"

A protester surnamed Lee said she joined the march because she felt that chief executive Carrie Lam is protecting the Hong Kong police from independent scrutiny amid widespread criticism of their use of force against mostly peaceful protesters.

"She says that protesters are coming out and causing chaos and destruction, but judging from the news, it's the police causing chaos and destruction," Lee said. "They use excessive force and just fire tear gas."

"So many people have committed suicide," she said, in a reference to several discoveries of protesters' bodies in cases that are widesly regarded as suspicious by the majority of protesters. "They say that these deaths aren't suspicious, but I don't think people in Hong Kong are going to believe that."

Hundreds arrested

As the protest march made its way through the city's streets, taking up several lanes of highway, police declared it over, apparently expecting around a million people to disperse in a short time.

Riot police then moved into arrest hundreds of people who were trying to leave, bundling them aboard dozens of police buses that had moved in to collect them, according to live video feeds and social media posts from the scene.

Some 400 people were arrested in the Causeway Bay shopping district alone, Now TV reported.

Protesters also questioned whether two masked men dressed as protesters who smashed the glass doors of the China Life building in Wanchai were in fact protesters, as they fled on being challenged towards police lines, where they weren't arrested.

A pitched battle began at around 4.00 p.m. outside the Southorn Stadium branch of HSBC in Wanchai, where protesters broke doors and windows and damaged an ATM machine.

Riot police charged the area, spraying pepper spray and firing tear gas at the crowd, while frontline protesters hurled petrol bombs and debris back at police lines.

An international group of politicians and former top officials meanwhile wrote to Lam to express "grave concern" over police brutality over the Christmas period.

"We have been horrified to see reports of police firing teargas, pepper-spray and rubber bullets at close-range at shoppers, peaceful protesters and innocent by-standers on Christmas Eve, Christmas Day, Boxing Day and again on Saturday 28 December," the letter, which was posted on the website of the rights group Hong Kong Watch, said.

"We are profoundly disturbed by scenes of children and young people being severely beaten, and of rubber bullets being fired into people’s faces, acts which any ballistics expert would confirm presents a serious risk of injury or death," it said.

'Extremely unsatisfactory' explanations

It said attempts by Lam's administration to explain away the police use of force were "extremely unsatisfactory."

"We understand that the actions of a small group of protesters have been violent, and we do not in any way condone violence or vandalism,"

The letter, which was signed by Sir Geoffrey Nice, former chief prosecutor of Slobodan Milosevic, Bundestag lawmaker Martin Patzelt, former premier of Ontario Bob Rae and former Speaker of the House of Commons John Bercow, among dozens of others.

But the majority of protesters had been attacked aggressively by the Hong Kong police even while protesting peacefully, the letter said, adding that growing public anger had been "borne of your government’s refusal to listen to their real fears, understood by many around the world, for more than six months."

The letter also condemned the arrest and abuse of doctors, nurses and first-aiders at protest sites, reports of police in full riot gear with weapons patrolling hospital wards, insisting on accompanying doctors on consultations, demanding access to medical records, seeking access to hospital operation theaters and using ambulances to transport riot police.

It said the abuse of the press by police officers, as well as reports of torture, beatings and sexual abuse in detention were "profoundly concerning."

"We urge you to listen to the protesters’ demands," the letter said, calling for an independent inquiry into police violence, the release of all peaceful protesters and for Lam's government to consider democratic reforms.

Reported by Man Hoi-tsan, Lu Xi and Tseng Yat-yiu for RFA's Mandarin and Cantonese Services. Translated and edited by Luisetta Mudie.