Police said at least 11 people died and more than 50 people were injured at the street-corner election meeting of the Awami National Party in the Mominabad area of Karachi.
The ANP blast was the third militant attack in four days in what Dawn newspaper said was an attempt to keep secular parties from participating in the May 11 national elections.
The Taliban claimed responsibility for the ANP attack, the Dawn report said.
Earlier in the day of the ANP blast, ANP candidate Abdul Rehman Khan survived an attack when an improvised explosive device was detonated near his vehicle in Landhi area of Karachi.
ANP chief Asfandyar Wali condemned the attacks, saying "people know who are behind these barbaric acts and they want to force us out of the election process."
The ANP blast comes a day after a period of mourning was observed in Karachi for the six people who died in a bomb blast at the office the liberal Muttahida Quami Movement party.
Police said 12 people also were injured in the explosion for which the outlawed Tehrik-i-Taliban Pakistan claimed responsibility, Dawn reported.
Altaf Hussain, founder and leader of MQM, condemned the bombings at his party's office and the blast at the ANP, calling the bombers "enemies of the humanity."
In a statement on the MQM website, Hussain said the bombs were "a continuation of terrorism against moderate and enlightened political forces" and an attempt to disenfranchise people of their democratic, constitutional and legal rights.
He called for better protection of life and property.
Earlier this month Taliban militants in Sindh province killed an MQM candidate in Hyderabad.
Pakistan's troubled election campaigning was further complicated last week after a court in Rawalpindi placed former military leader and Pakistani President Pervez Musharraf under arrest on charges related to the 2007 assassination of Prime Minister Benazir Bhutto.
His government dismissed a 2010 U.N. report, which said the government failed to provide enough protection for Bhutto while she was campaigning in Rawalpindi.
The ruling is the latest setback in the retired army general's attempt at a political comeback, The New York Times reported.
Musharraf was under house arrest at his Islamabad-area villa in a separate case related to the emergency rule he imposed in 2007, when he detained and fired senior judges, the newspaper said.
Musharraf, 69, seized power in a coup in 1999 and ruled the country for nine years before losing an election in 2008. He went into self-imposed exile in Dubai and London in November 2008 but returned last month to fight for a seat in Parliament.
Another candidate in the May 11 elections is former Prime Minister Nawaz Sharif, 63 -- the man Musharraf toppled in the bloodless 1999 coup.
Also running for a seat is former international cricket star Imran Khan, who has been highly critical of U.S. drone operations against the Taliban.