Power failed across a large swath of western Cuba last night, plunging millions of people into darkness, including those in the capital of Havana and at the beach resort of Varadero. Other cities in central and eastern Cuba also had outages but for only brief spans.

There was no immediate word on what caused the blackout, which struck a little after 8pm (Cuba time) and was still mostly out in the capital more than three hours later.

Calls to the electrical system's headquarters were met with busy signals. Officials in the national government were not immediately able to offer an explanation.

State radio said power was gradually being restored but urged people not to use power-hungry appliances.

Lights came back on in at least one eastern Havana suburb after about two and a half hours. Havana's international airport reported that it had electricity and was continuing operations.

In the capital, home to about two million people, the lights went out in a 40km stretch from Havana's western residential neighbourhoods across the city's centre and Old Havana district and on to suburbs on the other side of the bay.

In the Vedado entertainment and business district, the only buildings with visible light were tourist hotels and upscale apartment towers, which have back-up generators.

Outages that began at the same time as Havana's were reported as far away as Santiago, the nation's second-largest metropolis about 740km away at the other end of the island, although the power there returned after only a few minutes.

Electricity was out for about 20 minutes in the central cities of Ciego de Avila and Santa Clara. The western city of Pinar del Rio was also without electricity.

Big blackouts were common in Havana in the 1990s when Cuba was dealing with an energy crisis and again in the middle of the last decade. But, while isolated outages still hit the city on occasion, blackouts of this scope have become rare.