The attacker punched Mr Lee Cheuk-yan in the face and also injured one of the lawmaker's assistants before Lee's staff managed to subdue him, broadcaster RTHK said on its website.

Police have arrested a 58-year-old man on assault and vandalism charges, the broadcaster said.

Mr Lee, 51, is general secretary of the Hong Kong Confederation of Trade Unions and a member of the Legislative Council.

The Hong Kong government website also lists him as a member of The Frontier, a prominent pro-democracy group.

A government spokesman said the assault was unacceptable, and police were investigating the case.

'Hong Kong is a peaceful and lawful society, and we do not accept such a violent act. We express our sympathy to Mr Lee,' the spokesman said.

The motive for the attack, a rare occurrence in normally law-abiding Hong Kong, was not clear.

In 2006, pro-democracy lawmaker Albert Ho was savagely beaten by three men wielding truncheons and baseball bats in a busy downtown McDonald's restaurant, an incident that shocked the city.

Four men were jailed over the attack last year. -- AFP