Top brass rushed back to their Wan Chai offices after the first attack at 3.30am, putting frontline officers on full alert and ordering officers to be extra vigilant at a mass rally on Sunday against the government’s controversial extradition bill.

Police said an Emergency Unit vehicle was approaching the junction of Arsenal Street and Lockhart Road outside headquarters when officers in the van noticed the black Mercedes parked at the intersection.

When they moved to intercept a man standing next to the Mercedes acting suspiciously, he turned around and threw a home-made petrol bomb that landed on the road, blackening one side of the police van. No one was injured and the man fled the scene in the car.

In the second attack, a petrol bomb was thrown by a man in the same black Mercedes as it passed by the Happy Valley station, hitting a corner of the fence protecting the building.

A security guard at a residential block opposite the station said he spotted a fire “as tall as a man” at around 2pm in surveillance footage at his post, but did not see anyone suspicious around.

After reviewing surveillance footage of the attack, police said the suspect had been driving the black Mercedes with covered plates on Sing Woo Road. The ­video showed the man stepping out of the vehicle, throwing the firebomb, getting back into the car and driving away down Sing Woo Crescent.

The vehicle used in the attacks was later intercepted near King’s Road in North Point. Police said the first suspect got out of the car and ran into a nearby building, where he was caught hiding on the 12th floor. He was described as having an arson conviction and a triad background.

The other three suspects were arrested after 6pm near a North Point residential address to which the vehicle was registered.

Police said that the four were believed to be friends and that their individual roles and motivation were being investigated.

After the Wan Chai attack, a senior police insider said that headquarters had been targeted and that he had never seen such a blatant provocation against the force in his 25 years of service.

“If the Emergency Unit van had not passed by and intercepted the attacker, we believe he would have hurled the Molotov cocktail at the building,” the source said.

“This attacker has declared war against the 30,000-strong force. He wants to embarrass the force but he is not going to win.”

A 40-second clip of dashboard camera footage taken from a passing car shows a man in black with a cap seemingly scouting the area and approaching police headquarters before turning around when the police vehicle appears.

The suspect is out of the frame when the petrol bomb explodes next to the police vehicle, and the Mercedes-Benz is seen speeding away.

Asked why officers at the scene had not given immediate chase, Superintendent Law Kwok-hoi of the Hong Kong Island regional crime said they were not sure if there were more explosives around their vehicle.

Lam Chi-wai, chairman of the Junior Police Officers’ Association, which represents two-thirds of the force, also condemned the attack as a “declaration of war” and a provocation on a scale he had not seen in a decade of service.

“It conveys an incorrect message to members of the public that police can be provoked. But I want to say that the force has all the power and capability to bring the culprit to justice,” Lam said.

“We condemn all unlawful actions, not to mention such an apparent declaration of war against the police force. I hope members of the public will stay rational.”