MANILA (REUTERS) - Each morning, a group of Filipinos rakes up piles of trash on the banks of one of the world's most polluted rivers, filling sacks in an endless pursuit to clean a waterway that is also a major source of ocean plastics.

These "river warriors" are a decade-old group of about 100 people who work to clear the glut of garbage floating or washed up along Manila's notorious Pasig River.

The 27km river cutting through the Philippine capital was once a vital trade route. But urbanisation and poor sewage planning have left the river all but dead.

"There's never a time without garbage here. It's unlimited," said Ms Angelita Imperio, a river warrior for six years.

The warriors wear rubber boots and elbow-length gloves, using rakes and handmade tools to scoop rubbish from stagnant waters in different locations.

The warriors started off as volunteers but now receive a basic income from a local government and operate in small groups at different parts of the river.

Ms Dexter Opiana, another river warrior with six years of service, says she and about 19 others work shifts of about seven hours and collect an average of 80 to 100 sacks a day, more during monsoon season.

Most of it is plastic wrappers, single-use sachets and packaging materials. Since the pandemic began, surgical face masks are sometimes mixed in among the other floating garbage.

Pasig's trash is not just a Philippine problem.

A 2021 report by Oxford University's Our World in Data estimated 81 per cent of global ocean plastic comes from Asian rivers and the Philippines alone contributes a third of that total.

The Pasig River alone provides up to 6.43 per cent of ocean plastic originating from rivers, the report said.

Despite the warriors' Sisyphean task, they are optimistic of better days ahead.

"This has been our advocacy, to have the river cleaned for the sake of our children, our parents, our nation and mother nature," Ms Imperio said.

Ms Joan Lagunda, assistant secretary at the environment department, said the authorities were coordinating with local governments to establish proper waste segregation practices and want informal settlers on the riverbanks to be moved.

Ms Marian Ledesma, a campaigner with Greenpeace Philippines, said the government should reduce single-use plastics and strengthen law enforcement on waste disposal and sewage.

"I've seen it done in other cities, in other countries, so I don't think it's impossible to revive and clean up Pasig River," she said. "It will need a collective action."