A major power cut hit Athens and parts of southern Greece, four weeks before it is due to host the Olympic Games.
The blackout disrupted many journeys (Photo: Cameron Hickey)
Greece's worst outage for years caused chaos on the roads, shut down the Athens metro and left hundreds stuck in lifts in offices and blocks of flats.
But Ministry of State spokesman Panos Lividas said that Olympic venues, as well as hospitals, "faced no problems as they have their own generators".
A malfunction at an electricity substation may have been to blame.
The blackout, which started shortly before 1300 local time (1000 GMT), may have been caused by a surge in use of air conditioning given the recent temperatures of 40C, some reports said.
The lights came back in central Athens 45 minutes later but in other parts of the city it was hours before electricity was restored, says the BBC's Richard Galpin in the Greek capital.
Sirens across city
The outage caused chaos in the metro, with passengers trapped in carriages underground.
Traffic ground to a halt as lights stopped working
The sirens of fire engines were heard across the city of nearly five million as hundreds of people stuck inside lifts called the emergency services.
Among the many stranded commuters and office workers was Transport Minister Michalis Liapis, who was demonstrating a new train link between the city and the airport to reporters.
Our correspondent says the worry is now that during the Olympics, demand for electricity will be even greater, given the hundreds of thousands of visitors expected in the capital.
Other affected areas were Larissa, around 250km (155 miles) north of Athens, parts of the southern Peloponnesus region and some Aegean Sea islands.