Power has been restored to millions of people after a Sunday evening blackout that cut the lights in much of western Cuba, including the capital Havana.

Electricity began to come back in the capital at about 1 a.m. and the entire city appeared back online by morning.

Officials have not said what caused the power outage, which struck just after 8 p.m. Sunday, other than that it started after an "interruption in a transmission line."

The blackout affected communities from Pinar del Rio, in the west, to Ciego de Avila and Santa Clara in the center of the country. The grid came back online from east to west, with neighborhoods of Havana returning in stages early Monday.