Jimmy Sham, convenor of the Civil Human Rights Front (CHRF), which has organized some of the largest protests during Hong Kong's four-month-long civil unrest, suffered wounds to the back of his skull and forehead, police said.
Photos from the incident showed Sham lying on the street covered in blood.
Hong Kong Police said Sham was attacked by four males "of non-Chinese descent" who were wearing masks and dressed in black, Chief Inspector of Mong Kok District, Ng Tak-nam, said in a press conference late Wednesday.
Some of the assailants set upon Sham with hammers while others used knives in the attack, which happened at 7:40 p.m. local time (7:40 a.m. ET) in Kowloon's Mong Kok district, Ng said. The attackers fled the scene in a private car and are still at large.
The CHRF said in a statement that Sham was conscious when taken to hospital where he received "emergency treatment." He is now in a stable condition.
It is the second time the activist has been attacked in less than two months. On August 29, Sham was attacked by masked men carrying baseball bats and knives in Kowloon district. On the same day, police banned a rally and march planned by the CHRF that weekend.
The group condemned the assault and called on police to conduct a thorough investigation to find the perpetrators. They are waiting for police approval for another protest on Sunday.
"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," the CHRF said in a statement.
Police condemned the "cruel and pre-planned violent incident" and said they had "assigned staff to safeguard Sham in hospital."
Amnesty International has urged Hong Kong authorities to launch a "prompt and thorough investigation into a brutal attack."
"Jimmy Sham was left bleeding on the street and has been hospitalized with head injuries. Even in the context of increasing attacks on activists, this incident is shocking in its brutality," Joshua Rosenzweig, Head of Amnesty International's East Asia Regional Office, said in a statement.
The incident comes a day after Hong Kong Chief Executive Carrie Lam was forced to abandon her annual police address after pro-democracy lawmakers repeatedly disrupted her speech and heckled her with calls to honor the demands of months-long anti-government protests.
Her address had been seen as a key opportunity for Lam to begin to remedy some of the perceived factors behind the unrest, such as widespread inequality and limited access to public housing.
Civic Party lawmaker Tanya Chan accused Lam of having "blood on her hands" and said she had no right to address lawmakers due to the chaos unleashed by Lam's now-shelved extradition bill with China.
Protests which began over the bill have now lasted more than four months, with the movement's demands expanding to include other issues such as an independent investigation into police brutality and democratic reforms.
While protests began peacefully, with hundreds of thousands turning out for anti-extradition bill marches, they have grown increasingly violent. Protesters regularly throw petrol bombs and bricks, and have begun vandalizing subway stations and China-linked businesses, as police respond with tear gas and water cannon.
On Wednesday, police arrested two people on suspicion of making explosives, days after a bomb was set off near a police car following another night of violent protests.
CNN's Stella Ko and James Griffiths contributed.
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