“I don’t understand why dye was sprayed on the religious building as there were not many people around,” said Mohammed Sadeque, 34, who came from Kolkata to settle in the city 12 years ago. But he appreciated fellow Hongkongers who helped to clean up the coloured mess afterwards.

The front, which applied for a march on Sunday but failed, condemned police’s use of a water cannon on the mosque, calling the action “totally unnecessary” as there were only “a few people” nearby.

Hong Kong Unison, which reported that some of its members were doused in the dye and injured while safeguarding the mosque, issued a statement saying it was “outraged by police’s unjustifiable and rash action”.

It demanded the force explain its actions for deploying the “stinging dye”.

Later in the evening, Ho Yun-sing, the district commander of Yau Tsim, visited the mosque to meet the imam for 45 minutes. It was unclear whether there was an apology.

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The tens of thousands of protesters began their illegal march equipped with banners and umbrellas from Salisbury Garden in the shopping district of Tsim Sha Tsui towards the West Kowloon terminus of the high-speed rail link, which was heavily guarded by police.

But before long, the protesters headed north all the way to Sham Shui Po along Canton Road and Nathan Road, as radicals forced open the shutters of shops and banks with mainland links and began trashing them and setting fires to several of them, including a Bank of China outlet.

Apart from the branches of Best Mart 360 – which had been target of vandalism after being accused of having ties with the “Fujian gangs” that had allegedly beaten protesters in the past – the Mong Kok branch of Chinese medicine group Tong Ren Tang was also smashed and its medicine cabinets ransacked. Protesters later returned to the medicine store to set it on fire.

Chung Hwa Bookstore in Yau Ma Tei, also seen as being mainland-linked, was another target, with protesters spray-painting and breaking through its shutters. The glass facade was smashed, and some books torn from the shelves.

Also in Yau Ma Tei, an elderly man who was accused of stealing at least three mobile phones from a trashed Xiaomi store was tied up by protesters who insisted they did not condone looting.

A tense stand-off also erupted near the Park Lane Shoppers’ Boulevard as protesters repeatedly hurled petrol bombs at Tsim Sha Tsui station and officers fired rounds of tear gas at them from inside.

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Protesters cut down a surveillance camera near a Yau Ma Tei MTR exit with an electric chainsaw, before setting it on fire. They then threw petrol bombs inside at least four metro stations through their closed exits – forcing the partial suspension of the Tsuen Wan line and the closure of at least 14 stations. The city’s railway operator the MTR Corporation has been the target of protesters who accused the operator of aiding the police’s clearance actions.

By evening, a bomb disposal robot – believed to have been deployed for the first time during the four-month protests – had also detonated a suspected explosive device inside a cardboard box placed at the intersection of Lai Chi Kok Road and Tong Mi Road in Tai Kok Tsui.

Protesters also set a traffic light controller ablaze, apart from the fires they started in the shops the broke into.

In Mong Kok, riot police fired tear gas and rubber bullets at the mob but later appeared uncertain whether to continue their actions. As they retreated, protesters threw bricks at their vehicles, breaking the windows. Police then returned to clash again with the mob, before retreating yet again.

The Hospital Authority said that by Monday morning it had received 27 patients with injuries related to Sunday's protests, aged between 11 and 73. Three remained in a serious condition, five were stable and the rest had been discharged. The Police Public Relations Branch said it did not have arrest figures yet.

Figo Chan Ho-wun, the vice-convenor of the front who pushed ahead the banned march alongside three pan-democrats in a personal capacity, claimed an estimated 350,000 people had taken part.

A 25-year old frontline protester, surnamed Tang, said demonstrators had resorted to more violent behaviour because peaceful actions had not worked.

“Carrie Lam Cheng Yuet-ngor just won’t listen,” the bank worker said, referring to the city’s chief. “We must continue with our fight.”

The police force condemned the mob, as it urged passers-by to stay away from the scenes of chaos.

“Fires set by rioters seriously affect personal safety of members of the public. Indoor fires can quickly get out of control and burn residents living above the shops,” the statement read.

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Police also said two men, aged 31 and 34, were arrested for “possession of an offensive weapon” in Tai Po and on suspicion of “supplying weapons” to protesters in Kowloon on Sunday. Weapons including 42 suspected petrol bombs and raw materials for creating paintballs were found in the boot of two vehicles – a taxi and a seven-seater car – during a search.

In a statement released just before midnight, the Hong Kong government strongly condemned the acts of “rioters who completely disregard law and order”.

Meanwhile, the city’s embattled leader Lam is set to leave for Japan on Monday to attend the enthronement ceremony of Emperor Naruhito, to be held the next day. She is expected to return on Tuesday.

Reporting by Phila Siu, Linda Lew, Kimmy Chung, Karen Zhang, Karen Yeung and Jeffie Lam