Khan immediately blamed the Muttahida Qaumi Movement (MQM) for the murder. The PTI is a reformist party, which promised to reduce corruption, an endemic disease, in the nation of 180 million people. The PTI finished the third largest party in the recently held elections in Pakistan.
The PTI chief said the death of Hussain had left the party in a shock.
Police said Hussain was shot dead outside her home in an upscale neighbourhood of Karachi, a stronghold of the MQM. Khan, in a tweet, blamed MQM leader Altaf Hussain directly responsible for the murder, saying the latter had openly threatened PTI workers and leaders.
He also slammed the British government for Hussain's death, saying he had warned them about the MQM leader, who is a British citizen, after he gave the threats.
The MQM leader is accused of murder charges in Pakistan and leads his party from exile in England. His party has been labelled as a terrorist organization by Canada, which it denies.