Former international cricketer Imran Khan has blamed the British government for not doing enough to prevent the murder of one of his political colleagues.

Zahra Shahid Hussain, the vice-president of Mr Khan's Movement for Justice (PTI) party, was shot dead outside her home in an upmarket part of Karachi on Saturday (local time).

Police say Ms Hussain was targeted by three men on a motorcycle. Her death was the latest following a bloody election campaign marked by more than 150 killings.

Mr Khan was quick to hold the head of Karachi's MQM party, Altaf Hussain, who lives in self-imposed exile in London, directly responsible for the killing.

He vented his anger over the murder in a series of Twitter messages.

He tweeted that he blamed the British government for his colleague's death, because he said he had warned it about Altaf Hussain.

"I hold Altaf Hussain directly responsible for the murder as he had openly threatened PTI workers and leaders through public broadcasts," Mr Khan said describing the killing as "a targeted act of terror".

"I also hold the British Govt responsible as I had warned them abt Br citizen Altaf Hussain after his open threats to kill PTI workers.

"I am shocked & deeply saddened by the brutal killing of Zara Shahid Hussain."

The MQM party, which represents the Urdu-speaking majority, has denied the claim.

Altaf Hussain condemned the attack in a statement and demanded a "judicial inquiry to get to the culprits".

In a video message released at a press conference in Lahore, Mr Khan said his party would hold a protest in Karachi against the killing on Monday.

He added the protests would extend nationwide on Tuesday, including in front of the election commission's offices in Islamabad on Friday.

"Zahra Hussain's sacrifice will not go waste," he said.

Senior police official Nasir Aftab says the motive behind the shooting was unclear and there was no immediate claim of responsibility.

"We are investigating all leads that we have got but cannot say at the moment whether it was an attempted street robbery or a targeted killing," he said.

The murder took place on the eve of a controversial partial re-run of Pakistan's general election.

Troops, police and paramilitary rangers backed up by armoured personnel carriers guarded the 43 polling stations in the NA-250 constituency in Karachi, following claims of ballot-stuffing there during the May 11 general election.

An election official said turnout was initially assessed at more than 40 per cent despite scorching weather and Ms Hussain's killing.

ABC/AFP