Nearly 2,000 people have been killed in recent years

The Prime Minister, Nawaz Sharif, said in a televised speech that he could not leave the provincial capital, Karachi, to the mercy of "crime and terrorism".

He said a security operation had already been started in the city - where more than 600 people have been killed this year.

The BBC Islamabad Correspondent Owen Bennett-Jones says widespread arrests are likely but previous attempts to control the violence have not succeeded.

Police have failed to keep control

The crackdown follows the assassination two weeks ago of Hakim Said, a former governor of Sindh, who had a reputation for being politically neutral.

The killing sparked a series of political, ethnic and turf feuds and the government declared its determination to find those responsible.

Mr Sharif said: "This was not the death of a man, this was a full attack on humanity in society and an attempt to tear up or souls, morality and culture. Karachi is demanding accountability for the blood of its sons."

He said investigations pointed to the involvement of the MQM - a Karachi-based party which represents refugees who came to Pakistan at the time of partition by Britain in 1947.

He has challenged the party - which was in the province's ruling coalition - to hand over the killers. The MQM has denied involvement and instead accused the prime minister of being a central figure in the killing.

On Thursday it ended its troubled alliance with Mr Sharif's Pakistan Muslim League party.

Feuding factions

Mr Sharif has dissolved the Sindh government

The violence in Karachi - dubbed the City of Death - has continued despite a huge security cordon and frequent pledges of a crackdown on political, ethnic and sectarian violence.

Karachi is Pakistan's biggest city and accounts for a vast amount of the country's trade and tax revenues.

Feuding between the MQM and a breakaway faction has been blamed for much of the violence, which killed 400 last year and 500 in 1996.