The competition at Teluk Senangin beach was the inaugural open water event organised by the Perak swimming community, with about 225 taking part.

Between 30 and 40 swimmers were stung by jellyfish drifting on the tide. A girl and a boy, Nor Damia Qalessya Rosli, 9, and Aqeel Zafran Norhalis Syafiq, 10, were sent to hospital with a possible allergic reaction to the jellyfish venom, according to the Manjung Civil Defence Force.

“The incident occurred after the organisers flagged off the swimmers at 8am today; at 8.15am all the participants had returned to shore in pain and headed to the medical base to receive treatment for jellyfish stings,” according to the defence force’s statement.

State executive councillor A Sivanesan said he was told that the majority of those who were stung were boys aged 11 to 15.