Imran Khan, the leader of Pakistan's PTI party, has blamed one of his political rivals for the killing of PTI vice-president Zahra Shadid Hussain.
She was shot dead outside her home in Karachi by gunmen on a motorcycle.
On his Twitter feed, Mr Khan said he was holding the leader of Karachi's dominant MQM party, Altaf Hussain, responsible for her death - a claim the MQM has strongly denied.
The BBC reports it comes as Karachi votes in a partial re-run of Pakistan's general election.
Police say the motive for Ms Hussain's shooting is unclear.
Mr Khan said Mr Hussain, who is in self-imposed exile in London, had "openly threatened PTI workers and leaders through public broadcasts".
He said he was also holding the British government responsible, as he said he had warned them about Mr Hussain.
Last week, police in London confirmed they were investigating complaints that Mr Hussain had broken UK laws by issuing threats in a speech he made the day after the vote.
In response to accusations of electoral fraud, he is alleged to have threatened his accusers with violence.
Mr Hussain says that his remarks were taken out of context.
Voting in Karachi on 11 May was disrupted by a bomb attack outside the office of the ANP party, in which 11 people were killed and more than 40 injured.
The bombing happened in the Landhi district of Karachi, where Taliban militants are known to be active.