The mob armed with sticks and bricks attacked the participants of the vigil, including women at the famous Liberty Roundabout here.
They also attacked mediapersons and damaged their vans.
Over a dozen people, including journalists, suffered minor injuries in the incident.
"The beard men reached the spot on four vehicles and attacked the participants with sticks. They also torn Taseer's portrait and smashed his pictures under their feet," a journalist, who was present during the vigil, said.
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An anti-terrorism court had awarded capital punishment to Qadri but the Islamabad High Court later suspended the sentence.
"As we were chanting slogans against the killer of Taseer, Taliban and extremists, the 'Lovers of Mumtaz Qadri' attacked us with sticks. They even did not spare the women present there and mediapersons," rights activist and a leader of Pakistan People Party, Abdullah Malik said.
Superintendent of Police Gulberg Rizwan Ahmed told PTI that police would identify the suspects through the media footage and would soon arrest them.
"The civil society members had not informed us about their programme otherwise in the presence of police this incident might not have happened," he said.
Earlier, participating in the vigil, Malik said, "Salmaan Taseer was a brave man who never shied away from raising his voice for the voiceless, the marginalised, the poor, and for women and minority rights. His martyrdom shook us all but it also made us realise that a brave man never dies, his legacy lives on."
Taseer's elder son Shahbaz was kidnapped by Taliban and he is still in their custody.