There was no immediate claim of responsibility for the blast.
Officials said the bomber probably intended to attack an anti-government protest, which had been scheduled for Monday.
The protest was called off after the government vowed to reinstate the country's top judge.
"We had prior information that there could be an attack during the 'long march'," Rehman Malik, Pakistan's interior ministry chief, said.
"The people who did this are trying to destabilise Pakistan."