The world's first attempt to retrieve thousands of tonnes of plastic floating in giant ocean gyres has begun with the launch of a huge floating barrier even as scientists question its likely effectiveness.

The Dutch non-profit group, The Ocean Cleanup, last weekend towed its 600-metre-long floating barrier into waters off San Francisco for a fortnight of trials.

It's then due to be dragged to the Great Pacific Garbage Patch, located between California and Hawaii, to start gathering some of the estimated 80,000 tonnes of accumulated plastic for disposal onshore.

The System 001 is planned as a 600-metre-long U-shaped floating barrier that will concentrate ocean plastic and enable it to be removed. Credit:The Ocean Cleanup

The U-shaped boom, with a three-metre skirt dangling beneath, will "act like a giant Pac-Man" as it drifts with the wind and waves, concentrating plastic debris for extraction, the organisation said.