Weekend revellers at Girgaum Chowpatty were in for an unpleasant surprise when thousands of small, jellyfish-like creatures started washing up on the shore on Sunday afternoon, stinging and causing allergies to swimmers.

Thousands of translucent blue creatures — 2 inches in length, with a 5-inch tentacle and a sail-like extension over the body — lay in clusters on the entire stretch from H20 Water Sports Complex to Malabar Hill.

While the D B Marg police confirmed that no severe injury has been reported, they have prohibited visitors from entering the water till the beach is safe again.

A constable at the police station said, “On Sunday morning, a swimmer was stung and it resulted in terrible rashes on his body. He was in severe pain, but went home after local fishermen administered first aid. To be on the safer side, we are stopping people from entering the water till the danger subsides.” The BMC has deployed staff to clean up the beach immediately, a D-Ward official said.

Although Dr Vinay Deshmukh, principal scientist of Central Marine Fisheries Research Institute (CMFRI) could not visit the spot immediately, he said, “From description, the creatures seem to be the Portuguese Man O’ War, a highly venomous oceanic animal. It belongs to the Siphonophore family, a class of marine invertebrates which is in fact not a single organism but a family.”

Deshmukh said that the creature’s stings can cause excruciating pain, and in rare cases can even be lethal. “The victim will suffer severe pain and swelling,” he said.

The Maharashtra Pollution Control Board has taken samples of the marine creatures as well as the sea water for tests. “We have collected samples of the fish as well as the water and will send them to the laboratory on Monday, ’’ said Dr Anant N Harshavardhan.