"We are awaiting details and for police investigation," Faisal Subvaari, a senior member of the Muttahida Qaumi Movement — the dominant political party in Karachi, told Reuters.
Farooq, former convener of MQM, was coming home after an evening walk when he was attacked. He received multiple stab wounds and later succumbed to injuries in a hospital.
Pakistani TV channels quoted sources as saying that his guard was the attacker.
Farooq is survived by his wife and two sons.
His body will be flown to Karachi after the investigation is over, according to sources.
Celebrations for the party’s president, Altaf Hussain’s birthday, were immediately canceled in London after the assassination news spread.
In Karachi, thousands of mourners along with party workers gathered outside the house of Farooq. Paramilitary forces were deployed in the port city of Karachi as tensions built up following the killing.
Farooq claimed asylum in Britain 11 years ago after more than seven years on the run from Pakistani police who accused him of involvement in murder and other serious crimes. He denied the charges.
Forty-five people were killed and hundreds were wounded in clashes in Karachi last month after MQM member Raza Haider was gunned down along with his bodyguard while attending a funeral.
The government blamed the Taleban and the banned militant group Sipah-e-Sahaba Pakistan (SSP) for the killing of the parliamentarian.
Farooq was one of several senior members of the MQM who have taken refuge in London. The party's top leader, Altaf Hussain, has lived in self-exile in the British capital since 1992.
The party represents the descendants of Urdu-speaking migrants from India who settled in Pakistan after the partition of the subcontinent at the end of British rule in 1947.
It's main rivals for power in Karachi, Pakistan's commercial capital, are ethnic Pashtun politicians.
MQM party workers were involved in bloody factional clashes and battles with the security forces in Karachi in the 1990s.
The chance of Farooq's death sparking factional violence in Karachi would appear to depend on whether British police establish any link to the MQM's rivals in Pakistan.
The MQM is a member of Pakistan's ruling coalition led by President Asif Ali Zardari's party.
