Half of India is affected by a second day of power cuts. Three regional electricity grids have collapsed affecting 600 million people in 20 states.
Areas affected include Delhi, Punjab, Haryana, Uttar Pradesh, Himachal Pradesh and Rajasthan in the north, and West Bengal, Bihar, Orissa and Jharkhand in the east.
Hundreds of coalminers were reported to be trapped underground because the blackout had immobilised lifts.
Hundreds of trains were stalled and traffic lights were not working, causing widespread traffic jams.
The BBC reports much of the railway network is now moving again, although a full service is not expected for many hours and there is a huge backlog to clear.
The massive failure - a day after a similar but smaller one - has raised serious concerns about India's outdated infrastructure and the government's ability to meet its energy needs.
Power Minister Sushil Kumar Shinde said he had appealed to states to stop trying to take more than their quota of power.
Power cuts are common in Indian cities because of a fundamental shortage of power and an ageing grid, but the collapse of an entire grid is rare. The last time the northern grid failed was in 2001.