India's energy crisis has spread over half the country as both its eastern and northern electricity grids have collapsed.
More than 600 million people are without power in one of the world's biggest-ever blackouts.
The power failure has raised serious concerns about India's outdated infrastructure and the government's inability to meet energy needs as the country aspires to become a regional economic superpower.
The outage in the eastern grid came just a day after India's northern power grid collapsed for several hours.
Indian officials managed to restore power several hours later, but the northern grid collapsed again this morning, said Shailendre Dubey, an official at the Uttar Pradesh Power Corp, in India's largest state.
The eastern grid failed at almost the same time, said SK Mohanty, a power official in the eastern state of Orissa.
The two grids serve about half of India's population.
Traffic lights went out across New Delhi. The city's Metro rail system, which serves about 1.8m people a day, has shut down for the second day in a row.
"We'll have to wait for an hour or hour and a half, but till then we're trying to restore metro, railway and other essential services," said Power Minister Sushilkumar Shinde.
"This is the second day that something like this has happened. I've given instructions that whoever overdraws power will be punished."
India's demand for electricity has soared along with its economy in recent years, but utilities have been unable to meet the growing needs.
Its Central Electricity Authority reported power deficits of more than 8% in recent months.
The power deficit was worsened by a weak monsoon that lowered hydroelectric generation and kept temperatures higher, further increasing electricity usage as people seek to cool off.
But any connection to the grid remains a luxury for many.
A third of India's households do not even have electricity to power a light bulb, according to last year's census.
