Grid failure left more than 300m people without power in New Delhi and much of northern India for hours today in the worst blackout for more than a decade.
The incident drew attention to chronic infrastructure problems that are holding back Asia's third-largest economy.
The lights in Delhi and seven states went out in the early hours, leaving the capital's workers sweltering overnight and then stranded at metro stations in the morning rush hour as trains were cancelled.
Electricity supplies were restored to Delhi and much of Uttar Pradesh, a state with more people than Brazil, by midday.
However, the states of Rajasthan, Punjab and Jammu and Kashmir were still without full power in the early evening.
Power Minister Sushilkumar Shinde said all power would be restored within hours.
Power shortages and a poor road and rail network have hampered the country's efforts to industrialise.
About 40% of Indians, 500 million people, lack electricity.
Grappling with the slowest economic growth in nine years, Delhi recently scaled back a target to pump $1 trillion into infrastructure over the next five years.
Major industries have dedicated power plants or large diesel generators and are shielded from outages but the inconsistent supply affects investment and disrupts small businesses.
Office blocks, hotels and large apartment buildings all use backup diesel generators.
Chaos reigned on Delhi's always-hectic roads this morning as traffic lights failed and thousands of commuters abandoned the metro. Water pumping stations ran dry.
"First, no power since 2am in the morning, then no water to take a shower and now the metro is delayed by 13 minutes after being stuck in traffic for half an hour," said 32-year-old Keshav Shah, who works 30km outside the capital.
"As if I wasn't dreading Monday enough, this had to happen."
The government's top economic planning adviser, Montek Singh Ahluwalia, said the blackout may have been caused by a mix of coal shortages and other problems on the grid.
"I've no doubt that this is the area that we need to show improved performance in, and we also need show a clear sense of what we are doing to prevent it," Mr Ahluwalia told Reuters at his office, where power had been restored some hours earlier.
He said the grid was better networked now than five years ago and power sharing was more common.
But blackouts lasting up to eight hours a day are frequent in much of the country and have sparked angry protests on the industrial fringes of Delhi this summer, the hottest in years.
At least 200 trains were cancelled with some stranded.
Authorities made restoring services to hospitals and transport systems a priority.
Mr Shinde blamed the outage on an incident near Agra, the home of the Taj Mahal, without giving details. He said repairs were being carried out fast compared to a similar grid outage in the United States four years ago.
"In 2008, there was a power failure in the USA. Their Federal Energy Regulatory Commission asked India for assistance and it took four days to restore the power," he told reporters.
India suffers a peak hour power deficit of about 10%. It has been made worse this year by a weak monsoon, driving demand from farmers pumping more water from wells.
The outage forced the shutdown of a nuclear power plant at Rawatbhata in the desert state of Rajasthan.
It will take about 48 hours to restart. Hydroelectric plants in the Himalayas and thermal power stations in the wheat belt of Punjab and Haryana were slowly returning to normal.
India has the world's fifth-largest coal reserves and relies on it for two-thirds of its power generation.
Disputes over land and environmental clearances and failure to invest in new mines and technology have held back coal output as demand rises.
Officials at Delhi's international airport said flights were unaffected.
Delhi's private power company, BSES, said northern India last not suffered such a major outage since 2001.
"This kind of breakdown shows that the system needs some big overhaul to increase credibility and increase the confidence in the system of India," said Jagannadhan Thunuguntla, equity head at Delhi-based brokerage SMC Capital.
"More homework needs to be done."