Tens of thousands of people gathered in Chater Garden in the city's Central district to take part in a rally opposing the Communist Party and calling for universal suffrage in the territory. Organisers had applied for a permit for a march, which was denied, but police did approve a rally as long as it stayed in one place. The crowd easily filled the park and protesters spilled into the nearby streets.
The turnout appeared to be the largest since an authorised protest on New Year's Day, when organisers said over a million people gathered to demonstrate but had their march cancelled by police midway through.
Police said some protesters began vandalising buildings and tossing water bottles at officers, prompting them to cancel the event. Police had sent community liaison officers to deliver the message to the rally's organisers, they said.
The plainclothes officers told the organisers that the rally was being called off, according to Ventus Lau, a member of the Hong Kong Civil Assembly Team, which organised the rally. Lau asked to see an officer's identification card, he said, but the officer initially resisted. Once he saw the officer's badge, Lau said, he attempted to clear the park, but the altercation had drawn a crowd of angry protesters and fighting broke out.
A small group of riot police entered the park in what looked to be an effort to assist their injured colleagues and clear the area.
A group of protesters chased the injured and bleeding plainclothes officers across the street, where they attempted to find safety in a building but were unable to enter because the doors were locked. Cornered, three of them were beaten by protesters with umbrellas and batons. One protester attempted to hit them with a brick. More police rushed in to disperse the crowd.
Moments later, police fired multiple rounds of teargas, sending protesters, and dozens of migrant domestic workers who gather in the area on their day off, fleeing. The three people beaten - two men and a woman - left in ambulances with their heads bandaged.
Backed by an armoured vehicle and water cannons, police raced down the nearby streets to clear protesters.
The police said in a statement that two officers from the Police Community Liaison Office were speaking with rally organisers when they "were suddenly surrounded and beaten up brutally by a large group of rioters with wooden sticks and other weapons."
"They were left with bloody head injuries. Such appalling acts are not to be condoned. The Police will endeavour to bring the assailants to justice," the statement added. Lau was arrested, local media reported.
The Hong Kong Government also condemned the attacks, calling them "outrageous," and accused "rioters" of exploiting peaceful public assemblies "as a cover for vandalism and attacks."
Police Commissioner Chris Tang was grilled last week by district councillors about the force's use of plainclothes officers, who often have no visible identification and further obscure themselves by wearing masks. One district councillor held up photos of plainclothes officers and asked Tang whether he himself could identify them, to which Tang responded that he could not.