More than 20 hours after the country's worst power failure, the three major electricity grids that had collapsed have been restored, Power Grid said on Wednesday.

The Northern, Eastern and North Eastern Grids -- that cater to over half of the country's 1.2 billion population -- were restored by around 0930 hrs today.

Triggering a massive power crisis, which brought most parts of the country to a grinding halt, the three grids collapsed at 1300 hours yesterday.

According to state-run Power Grid, 100 per cent supplies have been restored in all the three regions.

"System restored to normal state," Power Grid said in a statement.

Full power supply of 30,081 MW has been restored in the northern region, including Delhi.

Eastern region, which caters to 10 states and has the largest infrastructure in the country with at least 25,000 MW generation capacity, was the last one to be restored.

"In the Eastern region, 100 per cent or 11,052 MW supply has been fully restored," it said.

Till late last evening, only 50 per cent of the eastern region had enough electricity supplies.

The North East was the first one to be fully restored, last evening.

Even though the exact reason is yet to be ascertained, officials said that problems started in the Eastern Grid and there could also have been over drawals by some states.