A massive power outage has plunged northern India into darkness and stranded thousands of travellers on trains Monday after a supply grid tripped because of overloading, officials said.

Trains across eight northern Indian states and metro services in New Delhi were affected by the power outage that struck at about 2.30am local time.

Hospitals and emergency services were running on diesel generators.

People, woken from sleep, came out of their homes in New Delhi's sweltering heat as the entire city turned dark.

It could take up to 12 hours to fully restore the electricity supply in the eight northern states, including New Delhi, Punjab, Haryana and Rajasthan, the chairman of the state-run Uttar Pradesh state Power Corporation, Avinash Awasthi, said.

The power grid collapsed because some states apparently drew more power than they were authorised to do to meet the rising demand during the summer, adding that today's outage was the worst to hit the country in 11 years.

Blackouts are a frequent occurrence in many Indian cities because of shortage of power supply and an antiquated electricity grid.

Two weeks ago, angry crowds blocked traffic and clashed with police after power blackouts in the Delhi suburb of Gurgaon that houses many high-rise apartment blocks and offices.

With no power in some neighbourhoods for more than 24 hours, people erected blockades that paralysed traffic for several hours.

Transmission and distribution losses in some states are as much as 50% because of theft and connivance of employees in the power distribution sector.