Power returning after India's big blackout:
Power supplies have been largely restored in India after its worst-ever blackout, which left hundreds of millions of people across more than half the country without electricity.
The blackout, which lasted for well over 10 hours, began at 1.04pm (1734 AEST) on Tuesday and affected 19 states as well as the capital New Delhi, paralysing rail and road transport.
In a statement late Tuesday the state-run Power Grid Corp, which controls the country's transmission network, said electricity had been fully restored in New Delhi and the northeastern region.
Power had also been restored in up to 82 per cent in the northern region, and 65 per cent in the eastern region, it said.
Officials said the power grid serving the north of the country had collapsed in the afternoon, following a similar shutdown on Monday disrupted transport services and water supplies.
In what was described as a "cascading failure", the eastern grid then suffered a breakdown, followed by the collapse of the northeastern grid.
The grids together provide power to areas inhabited by more than half of India's population of 1.2 billion.
In addition, an estimated 300 million Indians, mostly in poor, rural areas, do not have access to electricity.
Tens of thousands of passengers were stranded at stations as the outages disrupted some 300 trains across the northern region.
Businesses, hospitals and other essential services ran on back-up power supplies. Flights operations were unaffected.
New Delhi's metro was suspended for two hours and staff evacuated the trains.
In the capital, and many other cities, traffic was severely affected as traffic signals tripped and caused major gridlock at intersections. Some 4000 traffic police personnel in Delhi were immediately deployed, the IANS news agency reported.
The eastern metropolis of Kolkata was not badly affected, as it is served by a private electricity board. But suburban train services were halted in the region.
The failure also trapped around 200 miners in a mine in West Bengal, as lifts failed but the men were rescued after emergency power supply was arranged, news channels reported.
Tuesday's blackout was India's worst in its modern history. While smaller power cuts are common, and riots broke out this year over power shortages, multiple grid failures have been rare. The last time the northern grid failed was in 2001.
With rapid growth, India faces a severe energy crunch, and experts have warned such major outages will become more common unless supplies are increased. With coal, oil and gas supplies insufficient to meet its growing needs, India is looking to increase its nuclear and solar energy supply.
"Patients suffering at hospitals, people stranded on roads and train stations. Is this a picture of India in the 21st century?," asked Gaurav Arora, a Delhi-based stockbroker.
"How can a country dream of emerging as a power when it has such an energy crisis threatening its economic growth?" he said.