The violence followed a Sunday afternoon rally through the centre of the town attended by tens of thousands.

Hours after the march ended, riot police pepper sprayed several protesters inside the New Town Plaza mall at around 10pm, while minutes earlier an angry mob had set upon an officer who had been separated from colleagues.

Police arrested 33 people outside Sha Tin Town Hall on suspicion of unlawful assembly.

A government spokesman strongly condemned the actions of some protesters.

“These violent acts will not be tolerated by society,” he said.

Officers had earlier tried to clear a large crowd of mostly young protesters occupying the intersection of Yuen Wo Road and Sha Tin Rural Committee Road.

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Amid a heavier police presence than at recent protests, officers warned the crowd shortly before 9pm they would start making arrests if people did not leave.

About 10 minutes later officers raised a red warning flag after protesters threw missiles at them.

Many protesters began leaving through the Sha Tin Centre shopping mall as police closed in but there were hundreds left.

Then chaos broke out.

Protesters pelted officers with helmets, bottles and other items from the first floor of the centre and adjacent Lucky Plaza as police marched on the street below. Police chased protesters into both malls and several people were subdued.

Some protesters had tried to leave via the MTR, which was reached through New Town Plaza, but were initially blocked by police. After a short stand-off, officers allowed people to head for the train station.

As they moved towards the station, riot police came from behind triggering a violent reaction from protesters.

Protesters hurled objects at officers as three separate clashes ensued.

Two policemen separated from colleagues were chased and beaten by protesters.

One officer was saved by a journalist who sat on him, protecting his head and shouting to protesters: “No! He is also a Hongkonger.”

As the trouble erupted, passengers on an MTR train in Sha Tin station would not allow it to leave to allow as many protesters to get on board as possible, fearing it would be the last one of the night and there would be no escape.

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Soon after, rail operator MTR Corporation announced that trains would not stop at Sha Tin station.

But 25 minutes later trains resumed stopping at the station and protesters began fleeing.

By around 11pm, police had left New Town Plaza and there were few protesters to be seen.

Civic Party lawmaker Alvin Yeung Ngok-kiu said it was “unreasonable” for police to charge towards the protesters who were simply leaving through New Town Plaza for the MTR.

“Police must explain why they did that,” he said.

New Town Plaza said in a statement that it had neither called police nor requested their assistance.

Earlier, a large crowd had gathered in a playground near Che Kung Temple for the rally.

The march, expected to begin at 3.30pm, started 20 minutes early because of the number of people there. The organiser said 115,000 people had taken part but police put the figure at 28,000 at the rally’s peak.

Marchers headed for the upper-level bus terminal near New Town Plaza, passing through various streets. The first batch of protesters reached the rally finishing point at the terminal at about 4.45pm.

Scuffles broke out as scores of protesters hurled objects including traffic cones and bottles at police at about 5pm.

Two witnesses said about 20 officers suddenly ran towards the protesters near the Sha Tin Jockey Club Swimming Pool on Yuen Wo Road without warning.

They said about three or four were pepper sprayed.

Dozens of officers in helmets and carrying shields were at the scene and in a stand-off with protesters.

Claris Wong, 19, said he was sprayed on his arms and was left with red marks.

“The others were sprayed on the face,” he said.

Other protesters, who put on helmets and masks and had umbrellas for protection, ran towards Yuen Wo Road, which was not on the official march route.

Hundreds then set up barricades across five lanes of the road after removing metal railings along the sides of the street. They also formed a chain to pass on supplies – including cardboard boxes to be used as shields covered in plastic wrap – to the front of the crowd.

At 6pm, dozens of officers marched closer to the protesters but kept a distance of about 50 metres. An officer using a loudspeaker warned the crowd to leave immediately.

Opposition lawmaker Eddie Chu Hoi-dick urged police not to use violence. “I am worried you will use violence on protesters like you did in Sheung Shui [on Saturday],” he said through a loudspeaker.

About 7.30pm, riot police advanced to within a few metres of protesters, many of whom were wearing safety helmets and masks, but the stand-off at the intersection of Yuen Wo Road and Sha Tin Rural Committee Road continued after 8pm.

Hundreds of protesters set up barriers with railings removed from elsewhere. Some in the crowd had chanted that it was time to leave to avoid clashes with police but many people paid no heed.

Protesters, among other demands, have called for the full withdrawal of the now-suspended bill, which would have allowed the transfer of anyone in Hong Kong to jurisdictions the city does not have extradition agreements with, including mainland China. They have no confidence in the mainland’s legal system.