For many years, the people of Karachi have witnessed numerous scenes of violence. Supporters of rival political and religious groups are being killed almost every day. All over the city it is possible to see body-bags lying in the streets.

Four MQM members were shot dead on Monday

The reasons for the violence are manifold. In the years after independence it was largely because of tension between native Sindis and Urdu-speaking migrants from India known as Mohajirs.

In the early 1980s, the violence was attributed to hostility between the Sunni and Shia communities of the city.

But much of it today is blamed on a feud between two groups known as the the Muttahida Quami Movement and the Mohajir Quami Movement.

The Mohajir Quami Movement was formed in 1984 by Altaf Hussain, who since the early 1990s has been in self-imposed exile in London.

The MQM was founded specifically to represent Mohajirs, many of whom settled in Karachi and the surrounding province of Sind.

But the MQM underwent an acrimonious split in 1991 for reasons that even now are still unclear.

Politics turned to violence

This ideological divide between the two factions rapidly became violent, and is believed by many analysts to be one of the key reasons for the numerous gun battles in Karachi today.

It is not uncommon for 50 people to be killed a week.

In the months immediately after the split within the MQM, there were two MQMs in Karachi: the faction led by Mr Hussain (known as the MQM Altaf faction) and the smaller break-away faction known as MQM Haqiqi (which in Urdu means the real MQM).

Last year Mr Hussain decided to rename his faction the Muttahida Quami Movement, distinguishing it from the MQM Haqiqi.

The authorities have repeatedly failed to tackle the violence

Soon afterwards the MQM Haqiqi decided to dispense with the word Haqiqi, which means that it is now known as the Mohajir Quami Movement.

The Muttahida Quami Movement and the Mohajir Quami Movement blame each other for the bloodshed on Karachi's streets, but both deny any involvement in what appears to be a series of tit for tat killings.

In most cases the culprits are not caught. Even when a suspect is arrested, the policy of the police is not to reveal any names.

Reports of killings and beatings in custody in Karachi are commonplace.

Security clampdown failed to stop killings

Successive governments in Pakistan have tried - and failed - to tackle the Karachi problem.

The administration of Benazir Bhutto attempted a security clampdown coupled with direct rule from Islamabad. But violence in the city escalated.

Miss Bhutto's successor as prime minister, Nawaz Sharif, tried a different strategy: a coalition with the Muttahida Quami Movement of Mr Hussain.

On the face of it, this was a clever political manouevre because Mr Hussain's party is the largest and most influential in the city.

But that relationship has come under considerable strain recently, because Mr Hussain says that factions within the Pakistani intelligence community are supporting the Mohajir Quami Movement in its sectarian war against his organisation.

Most commentators agree that Karachi's problems cannot be solved without strong action from the central government: yet given the strained relations between all the key parties of Karachi, that strong action is unlikely to be forthcoming.