Maharashtra's Power Minister Nitin Raut said maintenance work was on when the technical fault happened.
Mumbai and neighbouring areas witnessed a huge power outage that left large parts of the city without electricity, disrupted trains and also affected networks.
The power cut was due to "TATA's incoming electric supply failure", the Brihanmumbai Electric Supply and Transport (BEST) said on Monday.
Later, Adani Electricity said in tweets that its teams were trying to restore supply after a "major power grid failure" that had affect most areas in Mumbai.
"There is a major power grid failure due to which supply is affected in most areas of mumbai. As per grid safety protocols, Adani Power System has been able to sustain and islanded," said Adani Electricity.
"AEML is currently supplying to critical services in Mumbai. Around 385MW through AEML Dahanu generation. Our teams are working to restory the supply in the affected areas at the earliest. We regret the inconvenience caused," the company statement added.
In a video statement, Maharashtra's Power Minister Nitin Raut said maintenance work was on in Maharashtra State Electricity Transmission Company's 400 kv Kalwa-Padgha centre when the technical fault happened in circuit number 2, which was taking the entire load at the time.
"A major part of Mumbai and Thane has been affected due to this, power will be restored in 30-45 minutes," he said.
IS Chahal, the Municipal Commissioner of Mumbai, asked officers to get enough supply of diesel for at least eight hours so that there were no power failures in hospitals, especially in the ICUs. He also asked disaster control personnel to keep handy a mobile diesel generator, in case the crisis went beyond two hours.
On Twitter, many complained that the power cut had hit Work From Home and online classes.