The Tuesday attack in the city, which has been the scene of such deadly militant, sectarian and political violence for months, occurred as a large crowd was leaving after a public rally, Xinhua News Agency reported.
Pakistan's DawnNews reported at least four people died and 42 more were injured, some of them critically. The report quoted police as saying the explosive device was set off remotely.
Dawn said the outlawed Pakistani Taliban known as Tehrik-i-Taliban Pakistan claimed responsibility. It quoted a Taliban spokesman as saying the attack was directed against members of the Muttahida Quami Movement Party, part of Pakistan's ruling coalition, and warned of more such attacks against MQM workers and leaders.
In violence elsewhere in Pakistan, seven health workers, six of them women, were shot and killed Monday in northwest Pakistan.
Dawn quoted police as saying a vehicle carrying the Support With Working Solutions workers was traveling home from a school and health center in Swabi district when it was ambushed by gunmen.
Notable deaths of 2014 [PHOTOS]
Syrian Al Qaida group executes Lebanese soldier