At least 12 people died as heavy rain lashed the eastern Indian city of Kolkata and surrounding areas ahead of a major festival, flooding streets, disrupting transport and leaving residents stranded for hours, officials said on Wednesday.

Most of the rain, as much as 251.6mm (9.9 inches) in 24 hours, fell during the early hours of Tuesday and was the heaviest witnessed in the city since 1988, said HR Biswas, the regional head of India Meteorological Department (IMD) in Kolkata.

Police said nine people died in Kolkata, with most of the deaths due to electrocution. Two people drowned, they added.

The rains brought the state capital to a standstill, seriously hampering preparations for the upcoming Durga Puja - the biggest annual festival of Hindus in West Bengal state.

Many pandals, temporary structures built with bamboo and other material for the festival, and clay idols of the deities also suffered damage across the city.

Roads were submerged under waist-deep water in some areas, stranding vehicles and forcing commuters to wade through flooded streets.

Road, train, and air traffic were severely disrupted, with several flights and trains cancelled or delayed. Power outages affected multiple areas for hours, compounding residents' difficulties.

"I got stranded in my hotel as my flight got cancelled and the roads were all waterlogged," said Ranjan Panda, a water and climate expert.

Authorities said they have deployed water pumps to clear streets and railway tracks, with relief measures, including food distribution and emergency services, underway.

The IMD predicted more rain in the state and eastern India over the next few days due to the formation of a low-pressure area over Bay of Bengal.

The state government declared schools and other educational institutions closed on Wednesday and Thursday before the holidays for the festival take effect from Friday.

Officials said conditions will normalize by Wednesday evening while urging residents to remain cautious as water levels gradually recede in low-lying areas.

"This should not have happened after four hours of rain. West Bengal is not in a good condition," Sandip Ghosh, a local resident in Kolkata, told Indian news agency ANI, in which Reuters has a minority stake.