Gunmen have killed at least 21 people in Karachi in the past 24 hours, raising tensions in Pakistan's largest city as voters cast ballots Sunday for a replacement for a provincial lawmaker murdered in August.

Police said they were still investigating the motives behind the shootings, but many so-called "target killings" in Karachi in the past have been linked to gangs controlled by various groups including the city's two main political parties, the Muttahida Quami Movement and the Awami National Party.

"We cannot say whether all the killings were politically motivated or some gangs were involved because the killings took place in different parts of the city and were not confined to the area where the elections were being held," Karachi police Chief Fayyza Leghari said.

Raza Haider, the member of the provincial assembly who was shot by unknown gunmen in August, was a member of MQM, which claims to represent the Urdu-speaking descendants of those people who came to Karachi soon after the birth of Pakistan.

The group's main rival, the ANP, represents ethnic Pashtuns from Pakistan's northwest.

Both parties were competing for Raza's vacant seat, but the ANP announced Saturday evening that it would boycott the election. The shootings began around the time the ANP made its announcement.

"We have reports of at least 21 people who were killed in target killings since Saturday evening," Leghari said.

At least three of the killings did not seem to be politically motivated and were carried out during "other criminal offenses," Leghari said.

The shootings were reminiscent of the violence that followed Raza's murder. At least 37 people were gunned down in the days following his killing.

Police have arrested at least 20 people in connection with the most recent shootings, Leghari said.

(This version CORRECTS name of MQM politician killed in August)