Prosecutors in Russia have opened a criminal case against the country's power monopoly after a major blackout in the capital, Moscow, on Wednesday.
Commuters were left stranded after the outage
Public transport ground to a halt, Moscow's main stock exchange stopped trading and water supplies were hit.
The electricity outage was caused by a fire and explosion at a substation, the energy minister told parliament.
Calls are growing for the resignation of Anatoly Chubais, a former liberal politician who heads the monopoly.
Mr Chubais has been summoned for questioning by the Russian prosecutor-general's office.
Simple problems neglected
The United Energy Systems (UES) chairman apologised for the blackout and said he was ready to take responsibility.
About 20,000 commuters in the capital are reported to have been evacuated from underground trains stranded by the outage.
BBC Moscow correspondent Steve Rosenberg said restaurants and shops across Moscow were left without light and electricity. Hospitals were forced to switch on to emergency power supplies and some apartment blocks were left without running water.
The discomfort was made worse by the fact that it was one of the hottest days of the year so far with record temperatures, over 30C - and without the electricity to power air conditioning systems.
The Micex stock exchange stopped trading for two hours as many of its clients did not have power.
The blackout also affected regions neighbouring Moscow.
Officials ruled out the possibility of sabotage as the cause of the fire.
Russian President Vladimir Putin said the UES management had neglected simple grid repairs and growing consumer demands.
Mr Chubais is hated by many for his privatisation role
"I think it's possible to talk about the inadequate attention that the UES leadership has paid to the current activities of the company," he said on television.
Nationalist MPs in the State Duma said they were preparing a resolution calling for the resignation of Mr Chubais, who made many enemies in the 1990s for his key role in government when major state-run industries were being privatised.
Correspondents say that while the former deputy prime minister generally refrains from open criticism of the Kremlin, he has been one of the few public figures to condemn the prosecution of tycoon Mikhail Khodorkovsky.
Many of his political associates are in open opposition to Mr Putin.
Mr Chubais survived an assassination attempt in March, which some blamed on his recent plans to reform the energy sector.