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The Pakistan Tehreek-e-Insaf PTI's leader, former international cricket star Imran Khan, immediately blamed the killing on the Muttahida Quami Movement.
The MQM has a stranglehold on politics in Pakistan's biggest city, Karachi.
"Her death has sent shockwaves across the rank and file of the party," Khan said in a statement.Police said that two gunmen shot Hussain dead outside her home in an upscale neighborhood of Karachi, he said.
"I hold (MQM leader) Altaf Hussain directly responsible for the murder as he openly threatened PTI workers and leaders through public broadcasts," he 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."
Gunmen killed a senior female politician from a reformist party in Pakistan on Saturday night, the latest violent incident in a bloody election campaign and one that set off a war of words between two major opposition parties.
Around 150 people were killed in the run-up to national elections held last week, which handed a landslide victory to opposition leader Nawaz Sharif and his PML-N party.
It marked the first time an elected government replaced another one in a nation that has been run by military leaders for more than half its history.
Results from a handful of constituencies are still awaited amid accusations of vote-rigging.The shooting came hours ahead of re-polling in a key area beset by allegations of voting fraud.
It was not immediately clear who killed Zara Shahid Hussain, a senior member of the Pakistan Tehreek-e-Insaf party.
The PTI has promised to reduce endemic corruption in the nuclear-armed nation of 180 million people.
Denmark's Emmelie de Forest says she was 'amazed' to find out she had won the Eurovision Song Contest and admitted that when she was first told she "just didn't understand it".
Winner of the Eurovision Song Contest Denmark's Emmelie de Forest said she is “overwhelmed, surprised but happy”.
It's official. They cannot be beaten. DENMARK HAS WON THE 2013 EUROVISION SONG CONTESTFrom @Eurovision on Twitter:
Denmark has won the Eurovision Song Contest 2013 with singer Emmelie de Forest who scooped the top bauble.
Current standing is Denmark, followed by Ukraine and Azerbaijan but it is really close! Everything can change!From @Eurovision on Twitter: