At least four children were killed and four other people seriously injured after they were struck by lightning in Barasat in North Paraganas district on Sunday, police said.
Buildings with weak structures collapsed in several areas, including in the city, due to overnight downpour.
An elderly man was crushed to death after a portion of his house collapsed here. A two-year-old was killed in Murshidabad district when a wall collapsed. A similar death was reported in Bankura.
The rains triggered flood-like situation, affecting 12 districts, damaging nearly two lakh houses and rendering over a lakh people homeless.
Chief Minister Mamata Banerjee met district officials and postponed her upcoming north Bengal tour.
"Several districts, including North and South 24 Parganas, Murshidabad, Burdwan and Howrah, have been affected. Disaster management teams are working on a war footing," she said.
Banerjee said crops over 2.10 lakh hectares of farmland were destroyed across the state.
"The situation is getting worse with every passing day as more and more areas are getting inundated," the chief minister said.
"While the entire state administration is working to reach out to the affected, we urge all political parties and NGOs to assist in relief work," Banerjee said.
After several parts of the city got waterlogged, the Kolkata Municipal Corporation pressed into service over 300 pumps to drain out rainwater.
Meanwhile, the regional Met office has forecast heavy to very heavy rains at a few places in the Gangetic and sub-Himalayan West Bengal till Tuesday.
Fishermen have been warned not to venture out into the sea on the West Bengal coast.