Islamabad, Pakistan (CNN) -- At least 36 people were killed and 51 were injured over the weekend in Pakistan's southern city of Karachi in what officials are calling politically-motivated target killings, police said Monday.
Authorities expect the number of casualties to rise, according to Rafiq Gul, a senior Karachi police official.
Police say the violence is connected to the assassination of a prominent politician in August. Violence last month from similar target killings after another politician's death left at least 18 dead.
Pakistani Interior Minister Rehman Malik told CNN affiliate Geo TV on Monday that several arrests have been made in the killings and vowed that those responsible will be brought to justice.
"I assure my brothers and sisters in Karachi of strict action against the elements involved in targeted killings in Karachi," Malik said, according to Geo TV.
The killings began Saturday ahead of a provincial by-election to replace the assassinated lawmaker from the Muttahida Qaumi Movement, a liberal political party whose stronghold is Karachi. The MQM, one of the largest parties in Pakistan, is part of President Asif Ali Zardari's ruling coalition.
The Awami National Party, a political rival to MQM, boycotted the vote -- which the MQM won by a vast majority -- after demands to use election monitors were rejected.
"Before the election, MQM workers have been harassing people to vote for them," Sen. Zahid Hussain, a spokesman for Awami National Party, told CNN.
Both parties accused the other of responsibility in the killings.
"Among the killed, most of the people were Pashtuns who were killed by the MQM," Hussain said.
Qamar Mansoor, a senior leader of MQM, said eight of the party's members "and many other Urdu-speaking people were killed by the ANP."
The MQM mainly represents the Urdu-speaking people, who migrated to Pakistan from India during partition in 1947, while the Awami National Party represents Pashto speakers -- more of whom are moving to Karachi from northwest Pakistan for jobs.
MQM is wary of the potential strengthening of a second political or ethnic group in its stronghold of Karachi, and the two parties have been engaged in bloody battles for years.