MQM: The London-based party holding Karachi in its grip

The most powerful party in Pakistan's largest city, Karachi, is run from the quiet London suburb of Edgware. But the Muttahida Quami Movement's reputation is far from peaceful - its activists on the ground are key players in Karachi's constant political strife.

The MQM's party leader Altaf Hussain, who lives in self-imposed exile in London, is largely unknown in the UK, but in Pakistan he is famous as one of the country's most powerful and most divisive politicians, addressing mass rallies in Karachi by a conference telephone connected to loud speakers.

For years there have been claims that the MQM has been involved in violence - something the party denies - and for years the UK authorities have turned a blind eye.

But BBC Newsnight understands that now police in London are investigating the MQM for possible money-laundering and complaints alleging that Mr Hussain has used his base in the UK to incite violence in Pakistan.

The party denies the allegations that murders in Pakistan were ordered from London and says it is the victim of "malicious" propaganda. It declined to comment about the money-laundering investigation while police inquiries continue and stressed no arrests have been made.

Owen Bennett-Jones' report for BBC Newsnight was first broadcast in July 2013

Watch the MQM response here