Among the injured were Balochistan President Aurangzeb Kasi and his wife, the Associated Press of Pakistan reported.
Pashtoon Students Federation Vice President Malik Muhammad Qasim and a child were among those killed, the government-run news agency said.
The bomb, planted on a bicycle, went off as the Awami National Party, part of Pakistan's coalition government, began the rally.
No group immediately claimed responsibility for the attack but police said they suspect Kasi was the target because of his party's opposition to extremism in Pakistan, CNN reported.
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