The leader of one of Hong Kong's largest pro-democracy groups, Jimmy Sham was attacked with hammers.
Photographs on social media showed Jimmy Sham of the Civil Human Rights Front (CHRF) lying in the street, covered in blood, the BBC reported on Thursday.
Afterwards, the activist said he "remained committed to the ideal of peaceful non-violence".
Mass protests in support of greater democracy in the territory, which began in June, show no sign of abating.
The attack on Sham came hours after the territory's leader suspended her annual address after being heckled in parliament.
The CHRF said he was set upon by a group of up to five hammer-wielding men in the Mong Kok district of the Kowloon peninsula, and left with head injuries.
It added that he was conscious as he was taken to hospital, and was in a stable condition.
It was the second time Sham had been attacked since the protests began.
The CHRF linked the attack to government supporters, who are suspected of assaulting other pro-democracy activists in recent months.
"It is not hard to link this incident to a spreading political terror in order to threaten and inhibit the legitimate exercise of natural and legal rights," it said in a statement.
Jimmy Sham is a long-time political activist in Hong Kong, mostly known for campaigning for LGBT rights.
In the anti-government protests he has been one of the leaders of the Civil Human Rights Front, a non-violent protest groups behind several of the huge marches since June.
The group said it was applying for a police permit to hold a new march on Sunday.
Its recent requests have been rejected, while more hardcore activists have held protests in defiance of bans and clashed with police.
While still in hospital, Sham released a statement on Facebook, saying the attack "only enables me to connect even more" with fellow protesters.
He also thanked the police for swiftly arriving to help him and urged them to find who was behind the assault.ALSO READ | Video shows terrifying moment when Hong Kong Police shot 18-year-old protester in chest