A cloud of florescent powder was seen in the air above a waterfront public space near the headquarters compound of the Hong Kong government Monday evening, after a helicopter of the Government Flying Service buzzed above a crowd of about 40,000 attending an anti-China extradition bill rally.

A transportation chopper reportedly scattered a large quantity of dust when hovering over Tamar Park, a protest hotspot on the harbourfront. Organizers of the sit-in later found suspicious powder on the ground and many participants said it was extremely difficult to wipe off.

Protesters are demanding answers from city officials about the nature of the substance and whether it was intended to mark people for tracking purposes. Florescent substances shine brightly when exposed to an ultraviolet lamp.

Monday’s rally was part of a citywide class boycott and general strike calling for the full withdrawal of the extradition bill and an investigation into alleged police brutality.

What was believed to be florescent powder was found on the forearms and hands of attendees of a legal assembly on Monday. Photo: Weibo

Then it was rumored that the authorities wanted to identify and track those in attendance, in order for the police to take action against them when they attend future events, such as a rally to be held in Central on Tuesday evening.

Some protesters who had been hit with the powder were both baffled and incensed, questioning the motive as the Tamar Park assembly had been approved by the police.

Some said they would not be deterred by such a scare tactic, but are still worried that the powder could be harmful.

A spokesperson for the Government Flying Service categorically denied that its helicopter crew dropped powder on protesters, adding that a chopper on an airlifting mission carrying a patient from an outlying island did fly past Tamar Park Monday evening when approaching a nearby helipad. The police have also denied the rumor.

A file photo shows choppers of the Government Flying Service flying above Hong Kong’s Victoria Harbor. Photo: Handout

Last Friday evening, another helicopter was spotted above various locations throughout Hong Kong Island, Kowloon and the New Territories. It cruised for hours above police stations, government headquarters and even the Beijing’s liaison office. That was ahead of the mass demonstrations, road occupations and running battles between activists and riot police that again swept the city the next day.

The Hong Kong Police Force has adopted unconventional methods to track and round up troublemakers.

A jet of blue dye shoots from a police water cannon in Hong Kong’s Admiralty district to disperse protesters on Saturday. Photo: Facebook via AFP

The force has just taken delivery of two water cannons mounted on special vehicles. In addition to shooting water to keep protesters at bay, they can spew thick dye onto protesters in the street, enabling officers to track them down later as it is hard to remove without a special solvent.

One water cannon was deployed last Saturday in Admiralty. It shot blue dye onto protesters and journalists on a flyover near the office of the city’s top leader.