A Hong Kong pro-democracy protester allegedly assaulted by police -- in a beating captured on video -- said he had himself been charged Thursday with attacking officers, in a new twist to a heavily criticised case.

It is one year to the day since the attack on Civic Party activist Ken Tsang, footage of which was beamed around the world at the height of mass protests for free leadership elections in Hong Kong.

Tsang angrily dubbed the charges "ridiculous" Thursday. He was due to attend a police station later.

Video footage aired by local television network TVB showed a group of men hauling a handcuffed Tsang to a dark corner in a public park in the early hours of October 15 last year.

One stands over him and punches him, as three others are seen repeatedly kicking him, in a case that has rocked the reputation of the normally respected police force.

Since then little progress has been made in the case and the authorities have withheld the identities of the attackers, believed to be plainclothes policemen.

On Thursday a furious Tsang expressed his shock at receiving a call from police telling him that he was to be arrested and charged for attacking officers.

Tsang told reporters he faced one count of assaulting police and four counts of obstructing public officers.

It was not clear to what incident the charges relate.

Tsang blasted them as "groundless, over the top, ridiculous and against the facts".

He added it may be a "public relations tactic".

Tsang said the charges against him were a ploy to "dilute" the case against the police.

"All this should not be happening one year later... I feel that the government is just trying to make a stand by arresting me," he said.

Police said they had "no updated information" on the case.

Seven officers were arrested in November and suspended from duty over the attack -- but no charges have been laid and they have not been officially named.

Local media have reported that the officers will be charged this week, but police would not confirm that.

Tsang was granted permission in July for a judicial review challenging the decision not to identify his assailants.

He and his legal team argue that if the names are revealed to them they can pursue the case privately, in the event that the public case fails.

Tsang and his supporters gathered Wednesday night at the spot where he was attacked for a candlelit vigil against police violence.