Pakistani paramilitary troops raided the headquarters of the controversial and powerful Muttahida Quami Movement (MQM) party in Karachi on Tuesday, making about 20 arrests, seizing weapons and closing down the office, according to reports citing military officials. At least one person was killed during the raid.

A spokesman for the Pakistan Rangers told reporters that they had found a large cache of illegal weapons at the office, known as "Nine Zero," and that they had arrested several “criminals.” MQM leaders reportedly claimed that the Rangers had conducted a “siege and search operation,” and had arrested several of the party’s leaders, and raided the residence of the party leader, Altaf Hussain, and his sister, local news station Geo TV reported

MQM lawmaker and witness Saman Jafri told Al Jazeera that at least 15 people had been detained, and that police had opened fire with live ammunition during the raid on the office, killing at least one person. "All our workers were peaceful, and even us lawmakers were remaining silent while the operation was ongoing," he said.

The controversial MQM party, which is the fourth largest in the National Assembly of Pakistan, has been a coalition partner for several previous governments since the 1980s, and it has been accused of violence by international organizations and opposition groups. The United States Bureau of Citizenship and Immigration Services has alleged in the past that MQM was heavily involved in violent activities in the 1990s that destabilized Pakistan’s southern Sindh region where Karachi is located. The U.S., and Amnesty International and other groups have also accused the MQM and its breakaway factions of human rights abuses including torture and summary killings. 

“We showed you the weapons. Some illegal weapons are those which cannot even be imported into Pakistan. We suspect that these weapons may be those from missing Nato containers,” Rangers spokesman Colonel Tahir Mehmood told reporters. He said that, of those arrested, at least five people had a criminal record while some were reportedly awaiting the death penalty, Reuters reported.

MQM party workers have reportedly begun protesting the raid at the party's headquarters and the MQM Coordination Committee announced a “day of peaceful protest” across the country against the operation, Dunya News reported.

In February, the Rangers alleged that a deadly factory fire in 2012 in the southern town of Baldia, which claimed at least 258 lives was the work of the MQM and that the party was extorting money from the factory’s owners, Dawn reported. The MQM has denied the accusation. Last June, Hussain was arrested at his London home for alleged money laundering, BBC reported, an incident that sparked violent protests in Karachi.