Hong Kong descended into scenes of chaos on Sunday night, after riot police fired tear gas and rubber bullets at protesters and a mob armed with batons attacked people in a metro station.

The unrest came after tens of thousands took to the street for the seventh consecutive weekend amid an ongoing political crisis over a now-suspended extradition bill.

Footage posted on social media showed a marauding gang of masked men, wearing white T-shirts, attacking people wearing black, the colour of the protest, on the platform and inside train carriages at Yuen Long MTR station, in the north of the city – about an hour from where the day’s protest had taken place. Forty-five people were hospitalized following the violence in Yuen Long, with one person in critical condition, according to Hong Kong’s Information Services Department. Five people remain in serious condition.

The incident has raised fears that organized crime gangs, who are known to operate in the outer areas of the city, are becoming involved in the political crisis. In a statement Monday, police said they would not tolerate “violent behaviour” and were actively following up the two incidents “in order to bring the offenders to justice.”

In a press conference on Monday, Hong Kong’s embattled leader Carrie Lam condemned the violence and said the government would investigate the incident in Yuen Long.

Protesters and their supporters have criticized police for what they say was a delayed response to the violence in Yuen Long.

Democratic Party lawmaker and Yuen Long district councilor Zachery Wong Wai-yin told CNN that police drove away after he told them he was being threatened. He also accused officers of cooperating with local gangs, which are collectively known as Triads.

In an interview with local radio, Chief Superintendent John Tse Chun Chung of the police public relations branch, said police did not make any arrests because the two officers responding to the scene “did not have protective gears” and were waiting for reinforcements.

Speaking to the press alongside Lam on Monday, Police Commissioner Stephen Lo denied accusations law enforcement officials were working with gangs hired to attack protesters.

The violence in Yuen Long came as large numbers of protesters and police faced off in the city’s Central and Sheung Wan districts.

Sunday’s demonstration had begun as a peaceful march, with organizers estimating that more than 400,000 people had joined the rally, while police said 138,000 took part. Protesters, many wearing black, chanted “Free Hong Kong!” as they thronged through the city.

But, later in the afternoon, thousands defied police orders and marched beyond the designated end point towards the Chinese government’s liaison office. Protesters spray-painted slogans on walls close to the building and defaced its emblem before retreating when faced with a growing police presence.

Intermittent skirmishes continued amid chants of “Liberate Hong Kong,” as the crowd headed to the city centre. There riot police fired tear gas to disperse a large group of mostly young activists, who had charged police lines with metal barricades.

A CNN team witnessed police officers using rubber bullets against demonstrators and protesters throwing bricks at the police.

In a statement, the Hong Kong government said it “strongly condemns the protesters who blatantly challenged the national sovereignty by maliciously besieging and storming” the liaison office building.

Meanwhile, the Chinese Liaison Office condemned “radical demonstrators’ behaviours” and said the unrest “severely challenged… the authority of the central government.”

On Monday, Chinese state media ran editorials on the unrest, with the China Daily calling the protests a “dirty game being played by the political radicals” and Xinhua saying “escalating violent attacks and provocative acts have completely exposed these mobs and the forces behind them, who want to paralyse the SAR government.”

The Civil Human Rights Front, which organized Sunday’s march, is demanding an independent investigation into what it says are unnecessarily brutal police tactics, as well as the release of those who were arrested during previous street clashes.

The protests were initially sparked by strong public opposition to a proposed extradition bill. Critics fear that the bill, which has since been suspended, would allow citizens to be sent across the border into mainland China, to face trial in a system with a 99 percent conviction rate and a history of political prosecutions. On June 18, Hong Kong’s Chief Executive Carrie Lam said the bill was “dead” and there was “no such plan” to reanimate it.

For protesters, however, that is not enough. They are calling for the complete and formal withdrawal of the bill, as well as for Lam’s resignation and other democratic reforms in the city, which is ruled under a “One Country, Two Systems” arrangement with mainland China.