(CNN) -- Pakistani politician Zahra Shahid Hussain, who alleged vote rigging in Karachi, was shot to death Saturday on the eve of a highly contested re-run of balloting in her district, police said.
Hussain was killed in what sources described as an execution-style attack.
The killing came one day before voters cast ballots for the second time this month for one national and two provincial assembly seats in Karachi following allegations of vote rigging in early balloting.
Hussain was the vice president of the Tehreek-e-Insaf party, or PTI, the political group headed by former cricket star Imran Khan.
"I am totally shocked and deeply saddened, and still cannot believe anyone would kill such a gentle lady," Khan said in statement.
She was 67 and was an advocate for education, women's rights and welfare policy.
Khan, Hussain and the party made headlines following nationwide elections earlier this month, alleging vote rigging in Karachi and elsewhere.
Khan accused the leader of a rival party for the killing.
"I hold (MQM leader) Altaf Hussain directly responsible for the murder as he openly threatened PTI workers and leaders through public broadcasts," Khan tweeted.
But the MQM, one of Pakistan's largest and most liberal parties, denied the allegations.
Khan's accusations are "baseless, shameless and irresponsible," said MQM spokesman, Faisal Subzwari. "The kind of reaction we have seen from Imran Khan is immature to say the least and insensitive to the ground realities in Karachi."
Pakistan's election commission scheduled a new vote in Karachi, saying additional security measures were being taken to protect ballots.
A re-poll is taking place Sunday in Karachi after allegations of voter fraud during the May 11 nationwide election.
Political killings escalating
Pakistan has seen a rise in targeted killings in recent years across the political spectrum.
A gunman on a motorbike killed a parliamentary candidate and his young child in March in Karachi. Sadiq Zaman Khattak, a representative of the liberal, anti-Taliban Awami National Party, was leaving a mosque with his 4-year-old son when an assassin shot them both.
No one owned up to the attack, but the Taliban have threatened Khattak's party and have claimed responsibility for some deadly attacks against its members and other politicians.
On the day Khattak was killed, gunmen elsewhere ambushed a top prosecutor, Chaudhry Zulfiqar Ali. At the time, he was trying the case stemming from the death of former Prime Minister Benazir Bhutto, who had been assassinated while campaigning for her party. Ali was heading to court when attackers opened fire on his car as it passed through an Islamabad neighborhood.
The ambush also wounded his bodyguard, whom authorities assigned to protect him after he received threats from the Pakistani Taliban.
CNN's Aliza Kassim contributed to this report.