Rescue teams rushed to the scene and shifted the injured to Jinnah and General Hospitals.
The group was also behind the massacre of more than 150 people at a Peshawar school in December 2014 and the October 2012 shooting of Nobel laureate Malala Yousafzai.
At least nine policemen, including an officer, were among the wounded, Ashraf said.
The suicide attacker for Mall Road blast was traced back to Afghanistan while an accomplice was nabbed.
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Police said the blast took place after evening prayers and most members of the religious group had exited into the street, some making purchases nearby. "The condition of four policemen is stated to be critical", Rescue 1122 spokesman Jam Sajjad told PTI.
However, he did not give the exact age of the boy.
Ashraf said numerous wounded were police and three were hospitalized in critical condition.
Washington on Thursday offered a $5 million bounty for the Pakistani Taliban militant leader, Mulllah Fazlullah, a day after a suspected United States drone strike on a training camp in a remote part of Afghanistan killed his son and more than 20 other militants preparing to launch suicide attacks in Pakistan. A fire ball was seen after the blast at the Nisar Police Check Post.
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The Taliban group Tehrik-e-Taliban Pakistan claimed responsibility, saying it was a suicide attack.
"The PSL semi-finals will be held in Lahore as per schedule as we have made all security arrangements in this regard", Ashraf said.
This is the first explosion targeting the provincial capital in 2018.
Jamaat-ur-Ahrar had also claimed responsibility for an Easter Day bombing in Lahore a year ago that killed more than 70 people in a public park.
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