MQM leaders Dr Khalid Maqbool Siddiqui and Khawaja Izharul Hassan were present at the event when the attack took place but remained unhurt.
The Mehfil-e-Milad was organised by the MQM-P. The injured have been shifted to a nearby hospital.
Spokesperson for the MQM-P Aminul Haq confirmed six workers of his party were injured in the attack. "The MQM-P organises Mehfil-e-Milad every year."
Karachi East SP has confirmed that motorcyclists threw a cracker at the gathering.
DIG (Karachi East) Amir Farooqui told a private TV channel that the cops were deployed for the security at the event, adding that he didn't know if the organizers had sought permission to hold Mehfil-e-Milad in the area.