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

Hundreds of people have attended the funeral of the murdered vice-president of Pakistan's PTI party, Zahra Shahid Hussain, at a mosque in Karachi.

She was shot dead outside her home in Karachi by gunmen on a motorcycle.

PTI leader Imran Khan has blamed one of his political rivals for the killing.

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.

It came as Karachi voted in a partial re-run of Pakistan's general election.

Police are investigating whether Mrs Hussain's killing was the result of an attempted robbery or a politically motivated murder.

Doctors at Karachi's Jinnah Hospital have told the BBC their initial examination showed two bullet marks on her body. A full post-mortem report is expected to be released.

'Warnings'

Zahra Shahid Hussain Believed to be in her 60s

Retired university professor

Founder member of Imran Khan's PTI (Movement for Justice)

Central vice-president of the PTI

Former president of the PTI women's wing in Sindh province

Led a PTI protest in November against a Nato air strike which killed 24 Pakistani soldiers near the Afghan border

Mr Khan, a former captain of Pakistan's cricket team, 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 it about Mr Hussain.

Mr Khan tweeted his accusations from his hospital bed, where he is recovering from a back injury sustained during a fall at an election rally in Lahore.

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, Mr Hussain is alleged to have threatened his accusers with violence.

The UK Foreign Office said on Sunday it strongly condemned all acts of violence in Karachi, including Saturday's murder.

It said that London's Metropolitan Police Service was in the early stages of its investigation but would "respond appropriately to the concerns raised and will take any appropriate action".

Mr Hussain and his supporters say that his remarks were taken out of context. He has lived in the UK since 1991, saying his life would be at risk if he returned to Pakistan.

MQM spokesman Mohammad Anwar told the BBC that Mr Khan's comments were the "groundless, baseless" accusations of a man who had suffered a bitter, unexpected loss in the election.

He said there was only one reason that Mr Khan could have issued a statement of blame just minutes after the killing: "He is behind the murder. He is the mastermind."

Sunday's electoral re-run 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, which is taking place under tight security.

Voter turn-out appeared slow but steady, says the BBC's Shahzeb Jillani in Karachi.

The PTI is hoping to win the vote and make inroads in Pakistan's commercial capital, our correspondent adds.

Whatever the outcome of the re-run, it will not overturn the overall result of last week's vote, in which conservative leader Nawaz Sharif secured an unprecedented third term in power.

Violent campaign

The MQM is seen as a perpetrator - as well as a victim - of violence in Karachi, our correspondent says.

Analysis The arrival of Imran Khan's PTI party on Karachi's political landscape is unsettling for the city's biggest stakeholder, the MQM. The PTI's growing popularity is rooted in young voters, women and the educated middle class. These are more or less the same segments of the electorate the MQM claims to have represented for nearly three decades. The MQM started out as an ethnic party comprising descendants of migrants from India at the time of the partition. Over the years, the party has tried to shake its ethnic label. Still, it is widely seen as a party of Urdu speaking people (or Mohajirs) and is accused of ruling the city with fear and vote rigging -charges it strongly denies. Even if Imran Khan's PTI does well in Karachi's re-run of voting, the results won't change the big picture. However, it will give the PTI an influential foothold in Pakistan's commercial capital, something the MQM will see as a major setback.

Since the 1980s, it has won every election it has contested there.

But it also stands widely accused of ruling Karachi by fear and through vote-rigging, our correspondent says.

The general election on 11 May marked the first transition of power from one democratically elected government since the creation of the state of Pakistan in 1947.

However, the campaign was marred by violence in which about 150 people were killed across the country.

The MQM (Muttahida Quami Movement) is supported mainly by Muslim Urdu-speaking people whose families moved to Sindh province at the time of the partition of India in 1947.

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.