Fearing an outbreak of gastro-enteritis and other water-borne diseases in the wake of thousands rushing to Mahim beach in Mumbai to taste 'sweet' sea water, authorities on Sunday said they are keeping a tight vigil on all medical institutions and hospitals in the city.

"We are keeping a round-the-clock vigil on all hospitals in the city and suburbs, including peripheral medical institutions, for any case of gastro-enteritis as we have to be prepared for a possible outbreak of water-borne diseases," S J Damble, Joint Commissioner of the Municipal Corporation of Greater Mumbai, told PTI.

"Luckily, so far no case of gastro-enteritis has been reported. The acute cases should have come within four to 10 hours of drinking the water and there is no report of any chronic case," he said.

Thousands of people have thronged Mahim beach since Friday night following reports that sea water there had turned "sweet". However, the MCGM and Maharashtra Pollution Control Board on Saturday said that this was not an unusual phenomenon and cautioned people against drinking the sea water.

Despite the official explanations, people collected murky water in bottles and plastic bags to take it home due to their ignorance, Damble said. Children and women drank the 'holy' water on the beach.

MCGM's Chief Executive Health Officer J Thanekar said the corporation was yet to receive a bacterial report on the 'sweet water.'

Quoting officials of the National Insititute of Oceanography, Thanekar said, "It could be possible that contamination in the water might have been reduced due to the waters from Mithi river flowing into the mouth of Mahim Bay."