The PTI party chairman, Imran Khan, blamed Zahra Shahid Hussain's death on a rival party. Police told Pakistan's Dawn newspaper that she was killed during an armed robbery.
On the eve of a re-vote, a prominent Pakistani politician was shot and killed on Saturday.
Pakistan's Dawn newspaper reports that Zahra Shahid Hussain, who was the senior vice-president of Pakistan's Movement for Justice (PTI), was shot in the head during "an attempted robbery incident."
"According to police," Dawn reports, "robbers tried to steal Hussain's hand bag and opened gun fire upon resistance by the PTI leader outside her residence in Defence Housing Authority Phase-4."
Imran Khan, the celebrated former cricket player and the chairman of the PTI, blamed Hussain's death on Altaf Hussain, the leader of the rival MQM party.
"I hold Altaf Hussain directly responsible for the murder as he had openly threatened PTI workers and leaders through public broadcasts," Khan said in a tweet. He continued: "Zara Apa's murder has further strengthened PTI's resolve to stand up against criminals and terrorists!"
"Shortly after the incident, the MQM chief issued a statement of his own where he condemned the murder and demanded that the government immediately arrest the culprits and accord them the most severe punishment as per law.
"Imran said Zara apa, as she was known to all within the party, was an old and senior Party leader who was a mentor to the PTI youth and a committed ideological leader of the Party.
"'I am totally shocked and deeply saddened and still cannot believe anyone would kill such a gentle lady.'"
Pakistan had ordered a re-vote in 30 Karachi polling stations after reports of irregularities. The PTI party was among those complaining of voting irregularities.
As NPR's Julie McCarthy has reported, Hussain's PTI party has become a "third force in Pakistani politics."