(Adds MQM boycott of vote, security)
By Katharine Houreld and Syed Raza Hassan
ISLAMABAD, May 19 (Reuters) - An upmarket constituency of Pakistan's violence-plagued city of Karachi voted again under tight security on Sunday, a day after gunmen killed a senior politician from a reformist party in the district and a week after general elections.
It was not immediately clear who killed Zara Shahid Hussain, a leading member of the Pakistan Tehrik-i-Insaf (PTI) party of former cricket star Imran Khan.
Imran blamed the killing on the Muttahida Quami Movement (MQM) party, which has a stranglehold on the city. MQM leaders denied responsibility, condemned the killing and demanded a retraction from Imran.
The attack in the upscale Defence area, the family neighbourhood of assassinated former prime minister Benazir Bhutto, capped a bloody election campaign in which around 150 people were killed nationwide.
Last Saturday's elections handed a landslide victory to opposition leader Nawaz Sharif and his Pakistan Muslim League-Nawaz (PML-N).
But results from a handful of constituencies across the country are still uncertain amid accusations of vote-rigging. There is re-polling in a few others where security issues prevented voting.
Last week's election gave the MQM 18 of 19 national assembly seats in Karachi, Pakistan's biggest city. The constituency, known blandly as NA-250, where Sunday's re-poll is taking place is thought to be a stronghold of the PTI.
The re-vote took place at 43 of 180 polling stations. The MQM, which wanted re-polling of the whole constituency, boycotted the vote.
Whatever the result, Sharif's national landslide win is assured. But as Pakistan's financial centre, Karachi generates around half of government revenues and stability in the city is key to stability of the whole country.
For the first time, each ballot box was guarded by a ranger and a soldier inside and outside the booths to ensure security and no violence had been reported by early afternoon.
Police said that two gunmen shot Hussain on Saturday outside her home in Defence.
"Her death has sent shockwaves across the rank and file of the party," Khan said in a statement.
"I hold (MQM leader) Altaf Hussain directly responsible for the murder as he openly threatened PTI workers and leaders through public broadcasts," Imran, recovering in hospital from a fall during campaigning, added in a tweet.
"I also hold the British government responsible as I had warned them about British citizen Altaf Hussain after his open threats."
Altaf Hussain is accused of murder in Pakistan and leads his party remotely from exile in England. His party is designated a terrorist organization by Canada, a charge it strongly denies.
In recent days he gave a speech which many Pakistanis felt was an incitement to attack political rivals. British police are investigating whether or not it constituted a hate speech.
The MQM, a secular party, is locked in a battle with various rival contenders for influence in Karachi, including Pakistan's Taliban movement, which has sought to gain a foothold in various districts on the outskirts of the city in recent years.
Imran's election campaign electrified many Pakistanis, pushing the PTI from a marginal party to Pakistan's third largest.
Karachi, the nuclear-armed country's key port, is home to 18 million people. It typically sees about a dozen murders a day, a combination of political killings, attacks by the Pakistan Taliban and sectarian militant groups, and street crime. (Writing by Nick Macfie; Editing by Jeremy Laurence)