The police watchdog rejected an invitation from the force to observe officers' handling of the candlelight vigil on Tuesday.
The Independent Police Complaints Council said the vigil, to remember the Tiananmen crackdown in 1989, had largely been peaceful in the last two decades, so it decided there was no need for its members to be there.
It was the first time the police had invited the council to observe the event, Hong Kong Island senior superintendent Cheng Yiu-mo said.
The arrangement was intended to help council members understand police procedures, Cheng said.
"The police have been making efforts to improve their transparency, and inviting council members to observe the event is one of the many ways of doing so," he said.
A council spokeswoman said its members considered many elements in deciding whether to attend an event, including its nature, whether it had a record of creating complaints, and whether it was controversial.
"The vigil has been held for more than 20 years, and generally it's smooth," she said. "The activity itself is mature, so we think there's no need for us to be there."
Meanwhile, following complaints from the organiser in 2011 about the closure of a gate near Tin Hau station, the police said all gates at Victoria Park except one leading to a narrow section of a street would be open for participants to enter the park.
An access way for emergency vehicles would also be open for public access until 8.30pm, half an hour after the vigil began.
The alliance in 2011 accused the police of diverting the crowd to a narrow, distant entrance to delay participants and limit their entry.
Afterwards, a group of protesters marched from the park to North Point police station and clashed with officers. Eight were convicted of unlawful assembly.
This year, student group Scholarism plans to hold an exchange session after the vigil.
Cheng said if it was a separate event, the organiser should inform the police. But the force had not received information about any events that would happen after the vigil.