Security officials had cordoned off the area on Bedian Road in the bustling capital of Punjab province after the blast, which rescue officials said occurred just before 8:00 am local time.
"It appears to have been an act of terrorism," Punjab provincial law minister Rana Sanaullah said in televised comments, adding that two soldiers and two civilians had been killed.
Rescue officials and police confirmed the toll, with a police official telling AFP the census team had been the target of the attack.
Teams of enumerators backed by the military and security forces are carrying out Pakistan's first census in almost two decades, an enormous and highly charged task that could redraw Pakistan's political map one year before national parliamentary elections.
Shahnaz initially described the incident as a gas cylinder explosion, but said the operation was ongoing.
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