Factbox: Pakistan commercial hub Karachi gripped by violence

KARACHI (Reuters) - A suspected suicide bomber on a motorcycle killed 12 Shi'ite Muslims in a crowded bus in Pakistan's commercial capital on Friday, hospital and police officials said.

Hours later, the hospital where some of the 40 wounded were being treated was hit by a huge explosion. A senior police official told Reuters that several hundred people were present at the scene of the second blast.

Following are some facts about the city:

WHO LIVES IN KARACHI?

* Karachi is the capital of Sindh province and has a population of around 18 million.

* Mohajirs, the descendants of Urdu-speakers who migrated from India after the creation of Pakistan in 1947, are the biggest community and dominate the city's administration.

* Karachi is also home to the largest concentration of ethnic Pashtuns outside North West Frontier Province. According to some estimates, more than 3.5 million Pashtuns live in the city.

* It is home to the central bank and main stock exchange and is also the country's main industrial base.

* The country's two main ports are in Karachi and most foreign companies investing in Pakistan have offices there.

HISTORY OF VIOLENCE IN KARACHI

* Karachi has a long history of ethnic, religious and sectarian violence. It was a main target of al Qaeda-linked militants after the September 11, 2001, attacks on the United States when Pakistan joined the U.S.-led campaign against militancy, and foreigners were attacked in the city several times.

* One of Pakistan's worst bomb attacks took place in Karachi in October 2007 during a welcome-home rally for self-exiled former prime minister Benazir Bhutto. About 140 people were killed. Bhutto was assassinated in the city of Rawalpindi just over two months later.

* Things have been relatively calm over the past two years as militants have focused attacks on cities in the north and across the northwest. But a bomb at a minority Shi'ite Muslim procession that killed 43 people on December 28 fueled concern that the militants were expanding their fight to the city.

POLITICS OF KARACHI

* The MQM, which mostly represents Mohajirs, is the dominant political force. In a 2008 general election it won 17 of 19 National Assembly seats in the city, while the other two went to President Asif Ali Zardari's Pakistan People's Party (PPP).

* The MQM controls the city government and is also part of a provincial coalition government led by the PPP as well as the federal government. The PPP dominates in rural areas of Sindh province.

* The MQM has been a strong and vocal critic of the Taliban, and also says that some Pashtuns are involved in crime, such as land grabbing and sheltering the militants.

WHAT IS AT STAKE?

* Karachi is a major transit point for military and other supplies to Afghanistan for the U.S.- and NATO-led anti-insurgency effort. Any trouble can directly affect those supplies.

* As the commercial hub, any trouble could disturb industrial activity, which can have serious consequences for the country's economy. According to officials, Karachi contributes 68 percent of the government's total revenue and 25 percent of GDP.

* While stock investors are getting used to trouble in the northwest, violence in Karachi has a more immediate market impact.