Marauding bands of activists opened fire indiscriminately in the streets in the early hours of the morning in the run-up to the strike called by the powerful ethnic party the Muttahida Quami Movement (MQM).

Activists also set fire to dozens of cars and buses and set up road blocks.

The strike was called by the MQM to mourn the death of four of its supporters who were shot dead in the city on Monday.

Most banks and markets remain closed and roads in many areas are deserted apart from police and paramilitary patrols.

Many roads are deserted apart from police patrols

Ethnically-motivated violence has been escalating since the MQM split several years ago into two factions which have been waging a bitter and increasingly violent battle for control of the city.

It is estimated more than 700 people have been killed this year alone.

This was the first protest strike call by the party since it rejoined the government a couple of weeks ago.

But the MQM says its supporters are still being targeted and killed despite assurances by government leaders that action would be taken against the rival faction.

BBC correspondent Zaffar Abbas says the strike shows the MQM is still extremely strong and popular.

Events have also demonstrated the powerlessness of Karachi's administration to contain the violence.

The fresh troubles also sparked a bitter debate in the National Assembly in Islamabad, where several MPs blamed the provincial administration in Sindh for its failure to maintain law and order.

But the Minister for Interior, Shujaat Hussain, said the central government wants to avoid extreme measures like the imposition of Governor's rule in Sindh province.

The MQM represents Indian Muslims who settled in Pakistan after the subcontinent gained independence from Britain.