(CNN) - Police in Hong Kong have seized what is thought to be the largest ever cache of high-powered explosives uncovered in the city, in a raid Friday night on a suspected bomb-making factory.
The discovery comes on the eve of a series of high-profile protests planned over the weekend, amid high tensions in the semi-autonomous Chinese city over a now-suspended extradition bill.
Police have confirmed the arrests of three local men aged between 25 and 27 in connection with the raid at what officers called a homemade laboratory in the industrial area of Tsuen Wan.
Police uncovered 2 kilograms (4.4 pounds) of high explosives, 10 petrol bombs, corrosive liquids, weapons and metal poles at the property.
"It's the largest such seizure we have ever come across in Hong Kong," said Alick Mcwhirter, superintendent of the Explosive Ordinance Disposal (EOD) Bureau. Leaflets linked to anti-extradition bill protests were also found on site.
Police said they were still testing the raw materials, but believe triacetone triperoxide (TATP) was found at different manufacturing stages at the suspected laboratory. EOD officers carried out a controlled explosion Saturday morning and were expected to perform more throughout the day.
A powerful high explosive, TATP was used in the November 2015 Paris attacks, the March 2016 Brussels bombings, the 2017 May Manchester bombing and a failed bomb attempt by an Islamist extremist at the Gare Centrale in Brussels in 2017.
The preparation of TATP can easily result in accidental detonation if mistakes are made.
In a statement released Sunday morning, police said the Organized Crime and Triad Bureau raided a factory unit after receiving a tip that someone was renting the space to store dangerous and offensive weapons.
According to the statement, a 27-year-old local man, surnamed Lo, was arrested for allegedly possessing explosives. Officers also arrested two 25-year-old local men, surnamed Hau and Tang, also for allegedly possessing explosives.
All three are under investigation. They face a maximum of 20 years in prison if charged.
At a press conference Saturday, Superintendent Steve Li Kwai-wah, said the 27-year-old man was wearing a shirt with the logo of the banned pro-independence group Hong Kong National Front when arrested by police.
On Saturday, thousands of demonstrators gathered in support of the government and police at a scheduled rally in Tamar Park, in the city center. An anti-extradition bill march will take place Sunday.
Li said police were still investigating a possible motive and intended uses for the explosives. He did not say if extra police would be arranged for protests this weekend as a result of the raid.
Mass demonstrations calling for government reforms and democracy have become a familiar sight in Hong Kong over the past six weeks, as hundreds of thousands of people have taken to the streets.
The demonstrations were initially sparked by strong public opposition to a bill that would have allowed people in Hong Kong to be extradited to face trial in mainland China.
Though the bill has since been suspended, demonstrations have continued against a backdrop of increased acrimony between protesters and police.