The city of more than 16 million people has a history of political, ethnic and religious violence, and has long been a hide-out for al-Qaida and Taliban militants. Its stability is important for Pakistan because it is the country's main commercial hub.
Independent analysts say followers of all political parties in Karachi are heavily involved in criminal activities.
A provincial spokesman says at least 10 people were arrested, and police and military troops were dispatched, but gunfire can still be heard and fires are still being set.
