HONG KONG (WASHINGTON POST) - Gas masks, helmets, umbrellas, goggles: for anyone in Hong Kong, this is a checklist of essential protective equipment needed to partake in or document the anti-government protests that have plunged the Chinese territory into political crisis.

But to authorities seeking to choke off the movement, these items are "tools for attacking people".

So said Hong Kong police when they visited an industrial-equipment shop in the city's Kowloon region two weeks ago, according to the manager. The man, who would be identified only as Peter to protect his business, said about 20 officers came to warn him that at "this sensitive moment, you know how to do it. You have to be careful".

The message, he said, was clear: stop selling gear to protesters.

Peter is doing nothing illegal - gas masks and helmets are not prohibited nor controlled goods - and police did not respond to questions about efforts to squeeze supplies of protective gear.

But as protests intensify along with Chinese government warnings of dire consequences, authorities are clamping down on the sale and importation of items they regard as tools of resistance.

In turn, vendors and protesters are devising ways to circumvent equipment shortages and keep the front lines supplied.

"Nowadays, we work like smugglers," said Peter. "We have to hide from the government."

Gas masks have become coveted devices in the Asian financial hub, where demonstrators have faced off with police in violent clashes. Police have fired more than 1,800 rounds of tear gas since early June, when street marches began over an extradition Bill, now shelved but not fully withdrawn, that would allow suspects to be tried in mainland China.

"There is no stock on the market" of gas masks, said a 43-year-old shop owner, who asked to be identified as Ho. He said he pressed manufacturer 3M's official distributor in Hong Kong to restock, adding: "I asked them for 100 but they only gave me 30."

Visits by The Post to several other shops yielded similar tales.

As protests become part of daily life, even in residential areas, demand comes from beyond the front line.

"There are many people buying these products," said Ho. "People that are in suits even."

The scarcest items right now are full-face gas masks, followed by respirators and goggles. Helmets are relatively easy to obtain, but carrying them draws suspicion. Police regularly stop and search protesters, and recently arrested a student for buying 10 laser pointers, accusing him of carrying offensive weapons. He was later released without charge.

To combat the shortages, Mr Lee Ching Hei, 33, has a solution: pop-up shops.

He recently opened a business, National Calamity Hardware, employing a team of 10 to scour for and snap up gear in Hong Kong.

At big protests, he rents a stall nearby and sells packets containing an eye mask, respirator, gloves and helmet. The packet costs him about US$64 (S$89), but he sells on a pay-what-you-can basis. The student price is US$1.27.

"The shortage is already happening in Hong Kong," he said. "I'm using all the ways I can just to make myself a supplier."

At a recent protest, Mr Lee said, about 20 police officers came to his stall looking for "laser pens or other dangerous goods".

He packages his supplies in black plastic bags. That way, protesters would not get stopped on the streets, and if the police want to know what is inside, they need to buy it. He said he charges government staff about 1,000 times the student rate.

"This is my business, not the government's," he said.

Hong Kong police have taken increasingly aggressive action during clashes with protesters, whom Chinese officials have branded "terrorists".

In June, after they detained and searched 358 protesters, police said they found items such as box cutters, razor blades and scissors, suggesting a confrontation was planned. Officers also confiscated cable ties, shirts, masks and goggles.

"They were organised and prepared radical individuals," said Mr Lee Kwai Wah, the senior superintendent of the Organised Crime and Triad Bureau, according to the South China Morning Post. "The masks and shirts were used to hide their identities."

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Police also have characterised laser pointers - which protesters used to distract officers and deter surveillance cameras - as a threat, by demonstrating at a news conference how their light beams could burn a hole in a sheet of paper.

The police are not the only source of pressure. Peter, of the Kowloon hardware store, said Chinese officials have been pressuring mainland companies not to ship certain goods to Hong Kong.

"All the shops and suppliers in the mainland already know there is a restriction on supplying some products to Hong Kong," he said, adding that his supplier had been interrogated by police and security officials in recent days.

In the hopes of getting half of his shipments, Peter said he orders goods to 10 different addresses, and sources them from Japan and Taiwan. He said many of the orders are seized by Chinese Customs officials at the border. China Customs officials did not respond to a request for comment.

As shops in Hong Kong run low, protesters are turning online, with mixed success.

One protester, a postgraduate student who asked to be identified as Carl, 24, ordered two anti-stab vests from China through Taobao, an e-commerce platform, in July, after a pro-Beijing mob attacked protesters in a subway station. The first one arrived. But an order he placed later that month was cancelled after a delivery service rejected it.

Another protester, a 29-year-old who would only go by the pseudonym "M", helps supply his comrades via the messaging app Telegram. He recently tried to order five masks through Taobao. M said the shop rejected the order because the delivery service indicated it was "unable to ship masks, gloves, goggles or umbrellas" to Hong Kong.

"You need to know some people to give it to you," he said. "You cannot just get it."

The Post reached out to five vendors in China on Taobao, and all said warehouses in China that process shipments to Hong Kong were refusing to accept protest-related supplies destined for the territory.

Alibaba, the parent company of Taobao, did not respond to questions from The Post about the supply limits. SF Express, a Chinese delivery-services company, declined to comment, as did Deppon, one of China's largest logistics companies.

Chinese restrictions on protective equipment could technically violate World Trade Organisation rules, said Dr Bryan Mercurio, a professor at Chinese University of Hong Kong's law school and an expert in international trade law. But under the national security provision in WTO rules, it is "likely that China would call this an essential security measure taken in time of emergency and prevail in any dispute".

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The tactics indicate that Beijing sees the Hong Kong crisis as "an increasingly more dire situation", said China researcher Adam Ni at Macquarie University in Sydney.

Restricting gear is part of China's "multi-pronged strategy" to "put more pressure on the protesters".

For now, Peter is not worried about the pressure. "Hong Kong still has the Basic Law to protect us," he said, referring to the city's mini-Constitution. Aside from needing to run his business, Peter said he also supports the protesters, to whom he has given free helmets.

"This product is not attacking anyone, it's just for protecting. It's not right to forbid this stuff to import to Hong Kong," he said. "Hong Kong is Hong Kong. Hong Kong is not China yet."