It is stated in the case that a blast occurred on Monday in a fruit-laden truck parked in the premises of Bhatti Parking Stand, adding a huge cache of explosive material was inside the vehicle. At least four injured are said to be in critical condition.
Rescue teams reached the scene and shifted the injured to Mayo Hospital and Mian Munshi Hospital, where emergency was declared.
He maintained that police is investigating and nothing can be said about the nature of the explosion.
Sharif was initially scheduled to travel to Lahore from Islamabad via the famous Grand Trunk Road on Sunday, but the rally was postponed to Wednesday.
Authorities probing Monday night's truck blast claim to have made headway in the investigation.
Bomb blasts by militants are common in Pakistan, especially in tribal areas bordering Afghanistan.
"Due the impact of the blast, I fell on the ground".
An eyewitness said the explosion shattered the windows of nearby buildings. A number of bikes and cars were also damaged due to its intensity.
Lahore, the capital of Punjab province, is the power base of deposed premier Nawaz Sharif. In April, six people died and 15 were injured in another attack in Lahore's Bedian Road, according to local media.
Similarly, a suicide blast on February 13 had ripped through a camp of protesting chemists in front of the Punjab Assembly, leaving 13 people dead and 70 others wounded.