Dr Imran Farooq - a leading member of the MQM (Muttahida Quami Movement) party - was found with head injuries and stab wounds outside his home in north London at 5.30pm on Thursday.
A Metropolitan Police spokesman said: "When officers arrived they found an Asian male, aged 50, suffering from multiple stab wounds and head injuries. He was treated by paramedics at the scene but was pronounced dead at 18.37. The next of kin have been informed."
He added that there had been no arrests.
According to reports on the MQM website from 1999, Dr Farooq sought political asylum in the UK after a contract was taken out on his life.
He claimed allegations of criminal and terrorist activities against him were false and politically motivated.
In November 1992 Dr Farooq said he was wanted "dead or alive". "(This gave) license and impunity to every individual in Pakistan to assassinate me," he said.
Dr Farooq, who was Secretary General of the party at the time, said he spent more than seven years in hiding in Karachi, southern Pakistan.
He added: "It was impossible for me to remain in Pakistan due to the continued threat on my life and liberty."
