Media playback is unsupported on your device Media caption The shooting happened on the eve of a highly contested vote

A senior female Pakistani politician has been shot dead in the southern port city of Karachi.

Zahra Shahid Hussain was the senior vice-president of Pakistan's Movement for Justice party (PTI), led by former international cricketer Imran Khan.

She was killed by gunmen on a motorcycle outside her home in the city's upmarket Defence neighbourhood.

Her murder took place on the eve of a highly-contested partial re-run of last weekend's general election.

The reason for the shooting is unclear.

PTI leader Imran Khan took to Twitter to blame Altaf Hussain, the London-based leader of Karachi's dominant MQM party for her murder - a claim the party has strongly denied.

Analysis It's too early to say with certainty who killed PTI politician Zahra Shahid Hussain or why. But within hours of her murder, suspicion fell on the city's dominant party - the MQM. PTI leader Imran Khan pointed the finger at MQM boss Altaf Hussain, who lives in self-imposed exile in London. The MQM responded by threatening Mr Khan with a defamation case. The MQM is seen as a perpetrator - as well as a victim - of violence in Pakistan's largest city. Since the 1980s, it has won every election it has contested here. But it also stands widely accused of ruling Karachi by fear and through vote-rigging. In the past, MQM has clashed with other smaller parties and ethnic groups trying to challenge its grip on the city. This time, it is seemingly up against the PTI, which is popular among educated urban young voters fed up with crime and lawlessness in the city. Since Saturday's election, Mr Khan's party has been calling for a partial re-run of the election. The MQM is boycotting the vote on Sunday, which the PTI hopes to win.

Mr Khan said Mr Hussain had "openly threatened PTI workers and leaders through public broadcasts".

Mr Khan, a former cricketer, said he also held the UK government responsible, as he had already given a warning about the MQM leader's remarks.

The MQM condemned the murder and described Imran Khan's allegations as "immature".

'Tragic incident'

Pakistan's Dawn newspaper, citing police, said the shooting happened during an attempted robbery.

However, our correspondent says that reports of Ms Hussain being shot twice in the head raise suspicions that it was a targeted killing made to look like a robbery.

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

Ms Hussain was reportedly rushed to hospital but succumbed to her injuries on the way.

Pakistani President Asif Ali Zardari strongly condemned the murder, describing it as a "tragic incident".

Sunday's partial re-run of the vote in Karachi was ordered after Mr Khan's party accused the MQM of widespread vote-rigging and intimidation.

The MQM - which took most of the seats in Karachi - denies any irregularities and is boycotting the vote.

The authorities have decided to deploy troops at 43 polling stations in Karachi's NA-250 constituency where Sunday's vote is taking place. Voting was halted at the polling stations during the 11 May election because of alleged irregularities.

Karachi is torn by regular violence - much of it politically motivated.

Last week's general elections appear to have paved the way for the first transition from one elected government to another in Pakistan - a country prone to military takeovers.

Unofficial results suggest that the Pakistan Muslim League led by former Prime Minister Nawaz Sharif will secure a majority in parliament.