(CNN) -- Pakistanis cast ballots in Karachi on Sunday, a day after the slaying of a well-known political leader who had accused rivals of vote rigging.
As voters headed to polls, accusations flew over Zahra Shahid Hussain's death.
Sources said an execution-style attack on the eve of the election killed Hussain, a senior vice president of the Tehreek-e-Insaf party, or PTI.
The party made headlines after nationwide elections on May 11, alleging vote rigging in Karachi and elsewhere.
Amid the allegations, election officials held a revote Sunday for one National Assembly seat and two Provincial Assembly seats in Karachi, Pakistan's largest city. Unofficial results from the country's election commission showed that PTI candidate Arif Alvi won the National Assembly seat.
Former cricket star Imran Khan, who heads the PTI party, blamed the leader of the rival MQM party for Hussain's death, accusing him of making inflammatory speeches threatening PTI workers and leaders.
"I hold (MQM leader) Altaf Hussain directly responsible for the murder," Khan said in a Twitter post.
Officials from the MQM, one of Pakistan's largest and most liberal parties, denied responsibility and sharply criticized Khan.
"The comments are absolutely baseless and are totally without foundation, made out of frustration from a man who has lost the election," said Mohammed Anwar, head of international relations for the MQM.
Anwar said Altaf Hussain, the MQM's London-based leader, was the first person to condemn the killing. Khan's comments, Anwar said, show a man lashing out.
"Within minutes of the murder, he was making accusations," Anwar said. "How did he found out so quickly after the event and issue a statement?"
MQM leaders called for a protest against Khan's "hasty" allegations.
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 and killed a top prosecutor, Chaudhry Zulfiqar Ali. At the time, he was trying a 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.
Meanwhile on Sunday, family, friends and members of the PTI party attended Hussain's funeral. She was buried not far from her home in Karachi.
British police investigating
Since 1991, the MQM political movement's leader has lived in London. Altaf Hussain sought political asylum in the United Kingdom "because of an attack on his life," according to a statement on the party's website.
London's Metropolitan Police Service said Sunday that it was investigating complaints about alleged comments made by someone associated with the MQM but did not provide additional details.
"We are in the early stages of the investigation and are assessing the information which has now been brought to our attention," the agency said in a statement. "We take all allegations of crime seriously and will respond appropriately to the concerns raised and will take action where appropriate."
The British Foreign Office condemned the Pakistani political leader's murder.
"We are deeply saddened by the recent violence in the city, including violence murderously directed against democratic political figures," a Foreign Office spokesman said.
CNN's Shaan Khan, Aliza Kassim and Joseph Netto contributed to this report.