A protest about a bad smell suspected of being tear gas in a Hong Kong neighborhood ended with the eye-stinging gas being fired by riot police after a restaurant was set on fire and a bank was vandalized on Monday night.

Hundreds had gathered in Tuen Mun in the New Territories on Monday night to protest against what they claimed was a smell coming from police testing tear gas at a nearby operations base.

In the afternoon, some residents in Leung King Estate and Tin King Estate started complaining on social media about a strange smell in the area. The smell was pungent and painful to their eyes, with many having running noses and shedding tears. They said their eyes felt like they had been irritated by chili peppers.

Police and the fire department received a number of reports saying there was a pungent and strange smell along Ming Kum Road and outside Tuen Mun North West Swimming Pool, near the two housing estates, Radio Television Hong Kong reported.

Many elderly people and children developed serious coughs while out on the streets. Paramedics were sent to treat two people who were feeling unwell. Some people distributed face masks to residents.

The strange smell was noticed at the Tuen Mun Ferry Pier, which is five kilometers from the housing estates.

Lawmaker Chu Hoi-dick said on Facebook that he received complaints from a Tuen Mun resident, saying her 3-year-old daughter had developed a rash on her neck and shoulder after leaving kindergarten at about 4pm, while the mother felt unwell and had a sore throat at her home. They suspected it was related to the mystery smell.

Online rumors surfaced in the afternoon, saying the police were testing tear gas or other substances at the nearby Tai Hing Operational Base in the same district.

Meanwhile, the Water Supplies Department temporarily stopped the water supply to six public housing estates in the district. Rumors went viral online, with many believing the mystery smell was a result of tear gas testing.

The Tai Hing Police Base in Tuen Mun is surrounded by a number of housing estates. Photo: Google Maps

The police denied the reports on its Facebook page, saying no tear gas had been used in the Tuen Mun area on Monday. After checking with the fire department, they said they did not know the source and cause of the strange smell.

The Environmental Protection Department said on Monday afternoon that Tuen Mun, Tsuen Wan, Tung Chung and Tsing Yi recorded a surge in nitrogen dioxide, ozone and sulphur dioxide, the Sing Pao reported. No reasons were given.

Meanwhile, according to a notice in residential buildings in Leung King Estates issued last month, the PLA Hong Kong garrison would hold drills from October 1 to 31 at the Castle Peak Firing Range in Tuen Mun. The garrison had yet to respond to inquiries from the Ming Pao Daily about any connections with the mystery smell and the drill.

At night, dozens of people responded to an online call, occupying a section of Leung Wan Street near the police’s operational base and chanting slogans to protest against the alleged gas testing.

Some people shone laser beams at the police operation base and set barricades on the road. At 9pm, riot police turned up with shields and weapons. They arrested at least one man on the spot.

Then some people attacked a nearby restaurant, which was operated by the Fulum Group. Protesters believed the eatery is linked to a Fujian clan involved in a number of attacks against pro-democracy protesters in the past few months. They threw a petrol bomb inside and set it on fire.

A nearby Bank of China branch was also vandalized.

Masked protesters vandalize the restaurant. Photo: RTHK

Riot police fired volleys of tear gas along the road and one cannister went into the seventh floor of nearby residential building Siu Hin Court.

A video showed a corridor filled with white gas. A resident surnamed Wong said he was standing in the corridor looking at what was happening downstairs. He saw riot police firing rounds of tear gas at a high altitude. Suddenly, a tear gas canister landed behind him.

Wong immediately told his neighbors to close their doors and he soon felt unwell. He criticized the police for firing tear gas at residential flats, which could cause serious harm to the elderly and children in their homes.