By the time police officers arrived, the attackers had left the scene.

The Hong Kong Police Inspectors’ Association issued a letter to Cheung venting its disappointment. It said officers were caught between the government and protesters, and Cheung’s remarks had shattered officers’ belief in their duty.

“Over the past few months we have been violently charged and verbally abused. Off-duty we suffer online abuse, harassment and provocation. Even our family members were insulted and threatened,” the letter read.

“But the entire force still stood fast and carried out our duties. We did not hold back. We did not complain. Because we believe we are the last defence of the administration.”

“Your words have completely written off our efforts in maintaining law and order over the past few months, and written off our sacrifices, too. You have completely disappointed us.”

The association urged Cheung to meet its representatives as soon as possible.

Anonymous angry letters, accompanied by pictures of police warrant cards or badges, were circulating online, urging Cheung to step down and apologise to the force.

“Police were being blamed for whatever they did. Now you are making things worse,” one letter read.

“The government gave us up, after we defended them for nearly two months. We took bullets for the administration,” said an inspector who had been working since the first extradition bill protest broke out in June.

“I was in shock when I came across the apology. Is it fair to all of us? The government owes us an apology instead!” said the visibly emotional officer, who spoke on condition of anonymity.

The force’s personnel wing issued an internal memo three hours after Cheung’s remarks, saying Lo and other managers did not know about the apology in advance and would seek a meeting with Cheung as soon as possible, along with the four police associations.

“The commissioner of police will attend the meeting himself together with the four associations’ representatives,” the memo read.

“We understand our officers in Yuen Long district have been criticised widely lately. But they still need to handle a very complicated mass public order event on Saturday. The director of operations will visit and meet with frontline officers in the district today.”

Earlier in the day, Cheung, who described the white-clad men behind the attacks as “rioters”, urged extradition bill protesters to demonstrate “peacefully and rationally” in the town on Saturday.

He said the government would take responsibility for its handling of the attack.

Beijing liaison office hits out at accusations it was behind Yuen Long attacks

“We are all very upset about the whole incident ... The government will not shirk its responsibility, and the police’s handling fell short of residents’ expectations. I am absolutely willing to apologise to residents for how it was handled,” Cheung said.

In describing the white-shirted attackers as rioters for the first time, Cheung’s remarks went further than those of Chief Executive Carrie Lam Cheng Yuet-ngor, who was widely criticised for refusing to apologise on Monday following the overnight violence, and for condemning protesters who vandalised the national emblem at Beijing’s liaison office before mentioning the Yuen Long attack.

On Thursday, the police’s acting New Territories North regional commander Anthony Tsang Ching-fo also stopped short of apologising, acknowledging only that “there was a bit of a gap between our operation … and the expectations of the public”.

Addressing protesters’ demand for an independent inquiry into the policing of recent protests, Cheung repeated the government’s position that established mechanisms – such as those within the Independent Police Complaints Council – could deal with the matter, but he said the government was in “reflective mode” and would look at ways to engage more with society.