BANGKOK, April 15 (Reuters) - Thai Prime Minister Prayut Chan-o-cha got a soaking on Friday (Apr 14) as a surprise participant in one of the world's biggest water fights, showing no signs of any dampened spirits from a slump in popularity just weeks from an election.

Dressed in the bright Hawaiian shirt traditionally worn at the Thai New Year festival Songkran, Prayut showed up unannounced at Bangkok's famous Khaosan Road and was the instant target of throngs of revellers armed with buckets and water guns.

Videos on social media showed the former army chief, who seized power in a coup in 2014, wiping water constantly from his head and face and taking fire from all angles as he strolled through crowds that cheered and whistled.

The 69-year-old general went on the offensive soon after, taking aim at partygoers from close range with a fluorescent pump-action water gun.

Prayut has been struggling in opinion polls ahead of the May 14 general election, with several recent national surveys showing his popularity at less than half of the frontrunner for the top job, Paetongtarn Shinawatra.