Pakistan soldiers raid MQM's Karachi headquarters
Security forces have raided the Karachi headquarters of Pakistan's fourth biggest political party, detaining "hardcore criminals" and seizing arms.
Col Tahir Mahmood said troops acted on information that the Muttahida Qaumi Movement (MQM) was hiding criminals.
Schools and businesses across the city closed down amid fears of a backlash.
The MQM, which has urged a national protest, has long been accused of using violence and intimidation to control Karachi - claims it denies.
At various times in its 30-year history, the MQM has been part of the national governing coalition.
But leader Altaf Hussain lives in self-imposed exile in London, where he is being investigated over money-laundering allegations.
And its control of Karachi is increasingly being challenged by ethnic gangs, says the BBC's M Ilyas Khan in Islamabad.
At the scene: Shahzeb Jillani, BBC News, Karachi
The MQM operates from the modest residence of party chief Altaf Hussain, and its adjoining buildings. The complex, known as Nine-Zero, has narrow, heavily guarded lanes that served as Mr Hussain's base until he went into exile.
For die-hard loyalists, many of whom were seen weeping after the raid, the sanctity of the neighbourhood has been violated. For critics of the party, however, Nine-Zero remains a no-go area from where the MQM has used the fear of guns and bullets to control this city of 20 million.
In the past, security forces have regarded the MQM as a force capable of turning Karachi into a war zone. But over the years things have changed.
Having spent more than a decade in government, the MQM is seen as weaker and somewhat directionless today. Its leader is under pressure from the British authorities as he faces a series of allegations, including money-laundering. He is also said to be suffering from health problems.
On the ground in Karachi, the MQM's muscle may be intact but it is believed that the army is managing to persuade the party to move away from its violent past.
After Wednesday's early morning raid, supporters and activists gathered outside the MQM headquarters and party leaders called for a nationwide strike.
Speaking to Pakistan's Geo News, senior MQM figure Farooq Sattar said the operation was "inappropriate and beyond understanding".
"MQM has itself always maintained its policy of zero tolerance against crime and terrorism, and it has always offered to co-operate," he said.
The MQM said paramilitary rangers had detained a number of its leaders including Amir Khan, a member of its central co-ordination committee.
Col Tahir told local media Mr Khan had not been arrested, but was being held for questioning to explain "the presence of criminal elements".
The colonel said the cache of weapons recovered included illegal arms that "cannot even be imported into Pakistan".
"We suspect that these weapons may be those from missing Nato containers," he said.
Party officials said the weapons were held legally and accused the security officers of shooting dead a party worker.
Other accounts suggested the worker may have been killed by a stray bullet fired in the air by unknown gunmen.
The MQM has appealed for a national peaceful protest.
What is MQM?
1984: Founded as the party of Urdu-speakers who migrated from India at the time of the 1947 partition, known as Muhajir
1988: Wins all seats in Karachi, becoming Pakistan's third largest party
1992: Party chief Altaf Hussain leaves country after an arrest warrant is issued in a murder case; army claims to have busted "torture cells" used by MQM activists to punish opponents
2004: Emerges as major ally of military ruler General Pervez Musharraf
2014: London police raid the home of Altaf Hussain, who still controls the party, and investigate claims of money laundering and murder