KARACHI (AFP) - Pakistani politician Imran Khan on Sunday blamed a rival political leader for the killing of a senior member of his party, who was gunned down outside her home in the violence-plagued city of Karachi.

The killing of Zohra Hussain, 59, vice president of the women's wing of Mr Khan's Pakistan Tehreek-e-Insaf (PTI) in Sindh province, came on the eve of a partial re-run in the southern city of the May 11 election.

Security was tight for Sunday's re-polling in one constituency following allegations of ballot-stuffing, with troops, police and paramilitary rangers backed up by armoured personnel carriers.

Ms Hussain was targeted by three gunmen on a motorcycle outside her home in an upmarket part of Karachi late Saturday, the latest killing following an bloody election campaign marked by more than 150 violent deaths.

Former cricket star Khan was quick to pin the blame on the Muttahida Qaumi Movement (MQM) party, which represents the Urdu-speaking majority, and specifically its boss Altaf Hussain, who lives in exile in London.

"I hold Altaf Hussain directly responsible for the murder as he had openly threatened PTI workers and leaders through public broadcasts," Mr Khan said on his Twitter feed, 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," he said, adding that he was "shocked and deeply saddened by the brutal killing".

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

Firdous Shamim, a local PTI leader, told AFP that Ms Zohra Hussain "was leaving her home for some work when three gunmen attacked her. She thought they wanted to snatch her purse and handed it over to them but they killed her".

Police said all three gunmen escaped after the attack.

"They shot her with one bullet near her chin and she could not survive," senior police official Nasir Aftab told AFP.

The motive behind the shooting was unclear and there was no immediate claim of responsibility.

PTI spokesman Jamal Siddiqui said a large number of party leaders and workers would attend Ms Zohra Hussain's funeral, which would take place in the city's Defence area Sunday afternoon.

Tensions have been running high between the PTI and the MQM, Karachi's dominant party, after Mr Khan's party accused it of widespread vote-rigging. The MQM has denied the charge and announced a boycott of the re-run.

The PTI and the right-wing Jamaat-e-Islami party have staged nationwide protests against the alleged irregularities.

The election commission ordered the partial re-run due to allegations of ballot-stuffing in a constituency known as NA-250 - a largely affluent neighbourhood of the country's financial capital.

Karachi last year suffered record violence linked to ethnic and political tensions.

The May 11 election marked Pakistan's first democratic transition of power after an elected government had completed a full term, a milestone in a country with a history of military coups.

Partial official results confirmed centre-right former prime minister Nawaz Sharif as the winner nearly 14 years after he was deposed in a coup.

His PML-N had 123 of the directly elected seats, with the outgoing Pakistan People's Party on 31 and Mr Khan's PTI on 26.