A well-known Pakistani rights activist and social media campaigner has been shot dead by unidentified gunmen in the port city of Karachi, police said Sunday.

Khurram Zaki, 40, was sitting at a roadside restaurant with a friend when four gunmen riding two motorcycles opened fire on him late Saturday, said Arab Mehar, a local senior police official.

"Khurram Zaki was killed in the shooting, while two others were wounded," Mehar told AFP, adding that an investigation has been launched into the attack.

Former journalist Zaki was an active campaigner for human rights and ran a website that condemns sectarianism.

He was known for his opposition to banned militant groups, including the Pakistani Taliban.

Zaki also led a protest against hardline cleric Maulana Abdul Aziz, from the red mosque in the capital Islamabad, calling for his arrest for inciting hate speech.

Pro-Taliban militant groups have carried out many attacks in the Karachi, Pakistan's biggest city, which is home to numerous ethnic groups.

Rights groups called for the government to take decisive action against those behind Zaki's killing.