KARACHI, Pakistan — Gunmen killed at least 37 people in Pakistan's largest city after a prominent lawmaker's assassination set off a cycle of revenge attacks, officials said Tuesday. Dozens of vehicles and shops were set ablaze as security forces struggled to regain control of Karachi. The southern city of more than 16 million 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.