A massive power outage struck Pakistan's largest city of Karachi early Thursday and left the entire city of more than 15 million people without electricity, a spokesman said. It was not immediately clear what caused the outage or when power would be restored, said Sultan Hassan, a spokesman for the Karachi Electric Supply Corp. Power from the state-run Water and Power Development Authority was suddenly cut, prompting the southern seaport city's entire electricity distribution network to shut down, Hassan said. "The power supply has come to zero. That is unprecedented," Hassan said. Outages on smaller scale are relatively common in Karachi, Pakistan's main port and one of the world's largest cities. Power cuts in the heat of last summer triggered rioting across the city.