A leading face of Hong Kong's pro-democracy movement was rushed to hospital covered in blood late Wednesday after being attacked by unidentified thugs with hammers, his protest group said.

The Civil Human Rights Front said leader Jimmy Sham was assaulted by four to five people wielding hammers in the district of Mongkok in what they described as an act of "political terror".

"He received a bloody head wound and was sent to Kwong Wah hospital," the CHRF said in a statement, adding Sham was conscious when paramedics arrived.

Photos posted online showed Sham in a red T-shirt, sprawled on the street in a pool of blood.

"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," CHRF added.

Sham is the main spokesman of the CHRF, a group which advocates non-violence and organised a series of record-breaking, peaceful marches earlier this summer that were opposed to a bill allowing extraditions to mainland China.

Hong Kong's four months of huge and increasingly violent protests were initially sparked by the now-scrapped bill.

But they have since snowballed into a wider movement calling for greater democracy and police accountability after Beijing and local leaders took a hard line.

Multiple pro-democracy activists have been attacked by pro-Beijing supporters in recent months, and Sham was also assaulted in August.

As the violence has escalated, hardcore pro-democracy protesters have also begun meting out their own street justice, beating people who vocally disagree with their goals or are viewed to be government loyalists.