Karachi, Pakistan's largest city, was tense on Wednesday after a pre-dawn raid by security personnel on the headquarters of the Muttahida Qaumi Movement, an official said.
The headquarters located in Azizabad neighbourhood was also the residence of MQM chief Altaf Hussain who fled Karachi in early 1990s and now lives in London as a British citizen.
Colonel Tahir of Rangers told media that the operation was launched at about 5.00 am and continued for two hours.
"About 15 people have been taken into custody and prohibited weapons were recovered. We also found weapons stolen from NATO containers taking shipments from Karachi port to Afghanistan," he said.
Tahir said that the intelligence-based raid was launched after reports that people involved in crimes were hiding there. He said no resistance was offered by those present.
The MQM with Karachi as its stronghold went livid at the raid and hundreds of its workers arrived at the headquarters to protest. The city was also partially closed due to fear of violence.
There were reports that Rangers personnel after breaking barricades leading to MQM headquarters cordoned off the entire area and searched the party's offices. They also raided adjoining houses and arrested many MQM activists and leaders.
Prominent MQM leader Faisal Sabazwari in a hastily-called press conference criticised the raid and asked Prime Minister Nawaz Sharif to intervene.
"Innocent workers of MQM are being killed in Karachi in the name of operation," he said.
He said MQM was a peaceful party and was being targeted for its struggle for the oppressed Urdu-speaking people who migrated from India to Pakistan in 1947 during the partition.
MQM announced a day of peaceful protest against the search operation.