Twelve people were injured in the explosion at an election office for the liberal Muttahida Quami Movement, Dawn News reported.
The MQM announced Friday would be a day of mourning.
Businesses and schools were closed.
The outlawed Tehrik-i-Taliban Pakistan claimed responsibility for the attack.
Wasay Jalil, a spokesman for the MQM, said the bombing would not keep the party from participating in May 11 elections.
MQM leader Altaf Hussain, speaking to a gathering of law enforcement officials and members of various organizations, charged the state had failed in its duty to protect citizens.