PARIS - Thousands of volunteers wielding nets and bin bags scoured coasts, parks and riverbanks across the globe Saturday, in a litter-picking drive highlighting the vast quantity of trash dumped worldwide, a day after mass international climate protests.

Campaigners took part in World Cleanup Day from Manila to the Mediterranean, as hundreds of thousands of people across the world take part in demonstrations and activities calling for urgent action on the environment.

Young people have been at the forefront of the movement, with masses of children skipping school on Friday for a global climate strike, which teen activist Greta Thunberg said was "only the beginning".

Some four million people filled city streets around the world on Friday, organizers said, in what was billed as the biggest ever protest against the threat posed to the planet by rising temperatures.

It kicked off a week of climate action called for by Thunberg, who was among several hundred young activists attending a climate summit at the United Nations on Saturday.

The World Cleanup Day on Saturday is an initiative that has got millions into the streets and cleaning up litter across the globe since it began just over a decade ago.

In France, volunteers posted images of their trash hauls on social media -- heaps of assorted waste, soft drink cans and plastic containers.

A climate protest on the streets of Paris attracted some 15,000 people, according to a tally by independent consultancy Occurrence, but the rally was marred by clashes between security forces and a relatively small number of troublemakers.

"The conditions for a non-violent march are not in place," Greenpeace, a co-organizer of the event, said on Twitter, as police fired teargas at protesters.

The mass cleanup is coordinated by the Let's Do It Foundation, which began life in Estonia and says that 18 million people from 157 countries participated in the global cleanup day last year. This year, it says more than 160 countries are taking part.

"Garbage is a global problem and it affects all the people in the world. It knows no state borders," said the organization's president Heidi Solba in a statement.

The group said about 30,000 people are participating in Estonia on Saturday in a drive focusing on small litter like bottle tops and cigarette butts.

Mart Normet, leader of the initiative in Estonia, said he drew hope from the enthusiasm of young people.

"When I look at the new photos on Facebook, I can see that the new generation is bringing a completely new attitude. Littering and damaging consumption are a thing of the past," he said.

In Vietnam's capital Hanoi, around 1,400 volunteers went searching for litter under the scorching sun.

"Although our actions are very small -- like cleaning trash from the sidewalk -- it could spread a meaningful message," 18-year-old Hoang Thi Hoan told AFP, as motorists zipped by on a busy street.

Friday's strike organizers 350.org said the youth-led rallies were the start of 5,800 protests across 163 countries over the next week.

UN chief Antonio Guterres met young activists in New York on Saturday, telling them: "There is a change in momentum... due to your initiative and to the courage, with which you have started these movements."

A landmark UN report to be unveiled next week will warn global warming and pollution are ravaging Earth's oceans and icy regions in ways that could unleash misery on a global scale.

The World Health Organization (WHO) said in August that the level of microplastics in drinking-water is not yet dangerous for humans but called for more research into potential future risk and a reduction in plastic waste.

Seventy-nine percent of the plastic ever made has ended up dumped, with little reused or destroyed despite recycling and other initiatives to curb use, a UN report from 2018 said. -AFP