The blast occurred near MQM's office in the densely populated North Nazimabad area of Karachi at about 9.30 am.
All six persons killed were MQM workers, leaders of the party said. Three of the dead were brothers, MQM leaders and local residents said.
The 15 injured people were taken to a nearby hospital.
The bomb was hidden in a car with government number plates.
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"The car had a government number plate and was parked close to the MQM election office in in Nusrat Bhutto colony of North Nazimabad," said Bukhari.
It could not immediately be ascertained if the registration number was genuine. The powerful blast damaged several nearby buildings and cars in the residential area.
A spokesman for the banned Tehrik-e-Taliban Pakistan claimed responsibility for the blast in phone calls made to TV news channels.
TTP had issued a threat few weeks ago, declaring it would target leaders and election rallies of secular parties like the MQM, Pakistan People's Party and Awami National Party.
Earlier this month, a MQM candidate in the upcoming general election was gunned down by Taliban fighters in Hyderabad, Pakistan's second largest city.
There have also been a string of bombings targeting the PPP and ANP in the country's restive northwest.
Caretaker Prime Minister Mir Hazar Khan Khoso, who was in Karachi for an official visit, condemned the blast near the MQM office. President Asif Ali Zardari too strongly condemned the blast.
In Hyderabad, SSP Hyderabad Saqib Ismail Memon said a security official was gunned down by two men riding on a motorcycle late night.