It was the second such attack on Jimmy Sham, head of the Civil Human Rights Front, since protests escalated in the Asian financial hub in mid-June and which, photographs on social media showed, left him in a pool of blood on the street.
"The authorities must promptly conduct an investigation into this horrifying attack...send a clear message that targeting activists will have consequences," said Joshua Rosenzweig, head of Amnesty’s East Asia regional office.
"Anything less would send a chilling signal that such attacks are tolerated by the authorities," Rosenzweig said in a statement on Wednesday.
As many as five masked men wearing black and believed to be of non-Chinese origin had attacked Sham with hammers and knives for more than 10 seconds in the gritty Mong Kok district on the Kowloon peninsula, police said late on Wednesday.
Police strongly condemned what they called a planned attack and vowed to investigate. Sham was in stable condition, a government spokesman said.
His group, which organized the million-strong marches in June, plans a march on Sunday in the neighboring district of Kowloon, but authorities have not yet confirmed it will be permitted.
"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 group said in a statement to Reuters.
Hong Kong has been roiled by more than four months of sometimes violent anti-government protests in its worst crisis in decades, posing the biggest popular challenge to Chinese President Xi Jinping since he came to power in 2012.What began as opposition to a now-withdrawn extradition bill has evolved into a pro-democracy movement fanned by fears that China is stifling freedoms guaranteed by a "one country, two systems" formula adopted when Britain returned Hong Kong to China in 1997.