A South Korean mayor who dumped a tonne of trash on a clean beach to give environmental campaigners something to pick up has said sorry for his deception.

Officials trucked in litter and waste that had already been collected from rubbish-strewn coasts, and scattered it all over the pristine sand of southwest Jindo.

The following day hundreds of volunteers scoured the shores and bagged up the garbage as part of an event to mark International Coastal Cleanup Day, a global event aimed at countering the scourge of sea-borne waste.

"We brought in waste styrofoam and other coastal trash gathered from nearby areas so the 600 participants could carry out clean-up activities," said Lee Dong-jin, Jindo county mayor, in a statement.

His office apologised for causing trouble and said the event was meant to "raise awareness about the seriousness of coastal waste".

Lee said all the litter had been retrieved and taken care of "100 percent". He said none had entered the ocean and there had been "no secondary pollution".

International Coastal Cleanup Day -- which this year fell on September 21 -- is one of the largest global movements for oceanic preservation.

Volunteers in around 100 countries took part this year, with beaches in Thailand, Hawaii, Israel and the Philippines also included.