A major grid failure caused a widespread power outage in India’s financial capital Mumbai and surrounding areas on Monday, the first such blackout in more than two years that stranded thousands of train passengers and delayed college exams.

The failure was caused by “technical problems” during maintenance work, the energy minister of Mumbai’s home state of Maharashtra said in a video message. Power has now been restored to many areas of the city, officials and witnesses said.

Mumbai’s international airport and the country’s two main stock exchanges located in the city, the National Stock Exchange and BSE, said they were operating normally.

“Power cut across the Mumbai Metropolitan Region due to grid failure,” the Brihanmumbai Electric Supply and Transport agency said on Twitter.

Adani Power Ltd and Tata Power Co Ltd, the other two power suppliers to the city of some 20 million, said they had also been affected by the outage.

The state government has asked the suppliers to provide uninterrupted power to hospitals, many of which are treating Covid-19 patients.

Hospitals and other institutions in India have over the years banked on emergency diesel power generators as a backup due to frequent outages caused by demand outstripping supply, though the situation is improving now.

Mumbai’s trains are generally packed to the full, moving more than seven million people a day to their offices and factories, but their services have been curtailed due to the Covid-19 pandemic and they are carrying far fewer passengers than usual.

Still, social media was splashed with pictures of people stranded inside unlit trains and in railway stations. Trains have now resumed, the government-run railways department said.

The Times of India said final-year online exams across Mumbai colleges have been postponed due to the blackout.