NEW DELHI: Power has been fully restored to three electricity grids which had failed in India, causing a blackout affecting more than 600 million people, a senior official told AFP Wednesday.

“Power has been restored fully across the northern, eastern and north-eastern grids,” Power System Operation Corporation chief S.K. Soonee told AFP, referring to the systems which collapsed Tuesday in an unprecedented outage.

Soonee said that power had been re-established overnight by engineers from PSOC, the state-owned company which manages the country's electricity generation system.

The three power grids collapsed from around 1:00 pm local time on Tuesday, and electricity was gradually restored during the course of the day.

The crisis affected 20 out of the nation's 29 states, an area home to more than 600 million people including the huge cities of New Delhi and Kolkata.

The government has announced an investigation into the cause of the crisis, which is a blow to the image of the world's second most populous nation as it strives for greater global economic and political clout.

States overdrawing electricity from the regional grids is thought to have caused the collapse. India has a large power deficit caused by surging demand which has outstripped the creaking national system's ability to supply.

An earlier blackout on Monday on the northern grid, described as the worst in a decade, hit in the middle of the night and left nine states without power for six hours.