Strong winds with speeds of about 65 miles (100km) per hour, lightning and heavy rains killed at least 18 people and injured 21 in the northern state of Uttar Pradesh overnight, police said.

"At least 18 people were killed in different incidents of house collapse and lightning," police spokesman Manish Awasthi told AFP in Lucknow, provincial capital of Uttar Pradesh.

The Press Trust of India (PTI), quoting officials, said nine people had been killed Thursday by lightning in various incidents in the eastern state of West Bengal.

In the western economic hub of Mumbai, the early-arriving monsoon caused travel chaos and brought back memories of last year's devastating floods that left hundreds dead.

One man drowned in the stormy sea off Mumbai and commuters were stranded for hours late Wednesday because of flooded roads and late-running trains.

The monsoon rains arrived several days earlier than expected and city officials had not yet completed anti-flooding measures, including finishing a project to dredge a river that runs through the city.

"It has taken one heavy rainfall at the beginning of the season for the authorities to have been caught unprepared," said an editorial in the DNA newspaper.

The city was lashed by intermittent rains for a second day Thursday, with low-lying areas becoming waterlogged and road and rail traffic being partially disrupted.

Weather officials said the heavy rains were expected to continue until Friday.

Environmentalist Anil Bhatia said Mumbai was better prepared for the rains this year after record rainfall in July 2005 swept away slum dwellings, cut electricity and severed the city from the rest of the country for a day.

Neglected drainage facilities and rampant illegal development, that left natural waterways blocked, were blamed for many of the problems.

Bhatia said decades of people flowing into the city of nearly 20 million, development and concreting of green spaces had all added to the city's drainage woes.

"Land use change has become so predominant, I would say it would take up to five years before we have a (anti-flooding) system in place again," he said.

The annual monsoon rains began sweeping through India in late May and the several months of rain are vital to the country's agriculture.