The attack targeted a campaign event organised in the city of Peshawar by the Awami National Party, which has been targeted by terrorists in the past over its vocal opposition to extremist groups like the Taliban.
The bombing came hours after the Pakistan military spokesman said there were security threats ahead of national elections scheduled for July 25.
"The attack killed at least 13 people, including (Haroon) Bilour," Peshawar city police chief Qazi Jameel said.
Officials said at least 54 people were wounded.
Haroon Bilour was one of the ANP's election candidates and belonged to an influential political family in the Khyber-Pakhtunkhwa province, of which Peshawar is the capital.
His father Bashir Bilour, one of the ANP's top leaders, was also killed by a suicide bomber in 2012.
"According to our initial investigation, it was a suicide attack and Haroon Bilour... was the target," police official Shafqat Malik said,
Police said the bomber struck when Bilour was about to address some 200 supporters.
Local television channels broadcast live scenes of ambulances arriving at the site and rushing victims to the hospital. Some people were seen crying.
No one immediately claimed responsibility for the attack, but there have been dozens of such bombings in recent years in Peshawar.
They have often been claimed by the Pakistani Taliban and their allies.
The city is considered a gateway to Pakistan's troubled semi-autonomous tribal regions, where many terror groups -- including al Qaeda -- operated until the government launched operations to oust them.
Terrorists have targeted politicians, religious gatherings, security forces and even schools in Peshawar.
