Tens of millions of Indonesians have been affected by electricity cuts due to technical problems at a series of power stations.
Homes and businesses were left in the dark across at least three provinces, including the capital Jakarta, and transport services were disrupted.
Officials said they were still trying to isolate the cause of the problem.
But they emphasised that the outage had nothing to do with recent power shortages caused by high oil prices.
Indonesia faces an increasing energy crisis, as demand outstrips supply. The government has called people to conserve energy, to reduce demand on the national electrical grid.
'Completely blacked out'
Operations at three major power plants on Java reportedly went down at 1030 local time (0330 GMT), causing the problem.
According to the Associated Press, the provinces of West Java, Banten and Jakarta were all affected, as were parts of the resort island of Bali.
More than 120 million people live in the affected region.
Thousands of police were mobilised in the capital Jakarta, to try to combat the problem, which officials hope to rectify within the next few hours.
"Jakarta was completely blacked out. The supply to the city had gone," said Mulyo Aji, a general manager at Perusahaan Listrik Negara, which operates the power network.
Jakarta police spokesman Tjiptono said the problem seemed to be "purely technical", and not the work of anyone attempting to sabotage electricity supplies.
"All vital projects are being protected and there is no sign of infiltration," Tjiptono told Reuters news agency.
The blackout caused serious traffic jams in Jakarta. Train services were also suspended, and some domestic flights were cancelled.
Many office buildings and hotels switched to running on generators, and school students were sent home.
A similar-sized problem occurred in September 2002. That power outage lasted two days.