PAKISTANI POLITICIAN IMRAN FAROOQ MURDERED IN LONDON
The Metropolitan Police said a 50-year-old man had died after suffering multiple stab wounds and head injuries.
The MQM has declared a 10-day mourning period and violence has been reported in Karachi, with cars being set alight and guns fired across the city.
It is understood nobody has been killed or injured.
Syed Shoaib Hasan BBC News, Karachi
Karachi is Pakistan's business capital, largest city and only operational commercial port.
It provides more than 50% of the tax revenue generated in the country and is literally Pakistan's lifeline.
A great deal of the economic problems stem from the fighting that took place here between the MQM and the government in the 1990s.
Although peace has since returned to the metropolis, sporadic violence has often put big pressure on the economy and the government of the day.
Although it remains part of the central government, the MQM has always had a separate policy for Karachi and reacts violently to any attack on its members.
In the past, this has led to a shutdown of all activities in Karachi, which is something Pakistan just cannot afford at the moment.
The metropolis is the driving force of Pakistan's economy, and its trade and industry is even more important, given that floods have wrecked the rest of the country.
Hundreds of party activists have converged on Mr Farooq's family home in the centre of Karachi, the largest city in Pakistan and the main base of support for the MQM (Muttahida Quami Movement)
Police in London were called to reports of a serious assault at 1730 BST on Thursday. Mr Farooq was treated by paramedics but declared dead at the scene about an hour later. No arrests have been made.
Raza Haroon, a member of the MQM central coordination committee, said: “He was a gentleman, a very, very soft spoken person with a lot of knowledge, and who was very outspoken as well.
“It's a very huge loss to the party to have lost a senior leader, in such a manner.
“This is an irreparable loss and a great tragedy for the MQM.”
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 1837.”
Mr Farooq disappeared from Pakistan in 1992 and is known to have been living in exile in London since 1999, when he claimed asylum in the UK.
He is understood to have been wanted by security forces and said in 1999 he had spent the previous seven years in hiding in Karachi.
Imran Farooq was attacked outside his home in Edgware
The former Pakistani parliamentarian was one of the founding members of the MQM, a former opposition party which is now part of the ruling PPP-led alliance.
The party said it had declared a 10-day mourning period in Pakistan and in its offices across the world.
Leaders said they expect to take Mr Farooq's body back to Karachi for burial after legal formalities have been completed.
The BBC's Shoaib Hasan in Karachi said he had spoken to party members and said “there's a lot of grief and a lot of sorrow going around”.
He said the activists who had headed to the dead politician's Karachi home appeared grief stricken and angry and many were weeping openly.
Hasan added: “I spoke to an MQM leader who was at the home of Imran Farooq in Karachi with his parents. They said that they are relying totally on the Metropolitan Police, that they have great faith in the Metropolitan Police.”
He said Mr Farooq was in essence the party's deputy leader and added he had not returned to Pakistan since his arrival in England in the 1990s.
Our correspondent said the MQM had had a “violent” past.
In 1999 Mr Farooq told the BBC charges against him in his home country were politically motivated.
At the time he said he intended to campaign against the Pakistani government of the day from exile in Britain.