Representatives from more than 170 nations will meet in Nairobi from Monday to negotiate what concrete measures should be included in a binding worldwide treaty to end plastic pollution.
Marchers waved placards reading "Plastic crisis = climate crisis" and "End multigenerational toxic exposure".
They chanted "let polluters pay the price" as they walked slowly behind a ceremonial band from central Nairobi to a park in the west of the capital.
Nations agreed last year to finalize by 2024 a world-first UN treaty to address the scourge of plastics found everywhere from mountain tops to ocean depths, and within human blood and breast milk.
Negotiators have met twice already but Nairobi is the first opportunity to debate a draft treaty published in September that outlines the many pathways to tackling the plastic problem.
The November 13-19 meeting is the third of five sessions in a fast-tracked process aiming to conclude negotiations next year so the treaty can be adopted by mid-2025.
At the last talks in Paris, campaigners accused large plastic-producing nations of deliberately stalling after two days were lost debating procedural points.
This time around, the sessions have been extended by two days but there are still concerns a weaker treaty could emerge if time for detailed discussion is swallowed up going in circles.
Global plastic production has more than doubled since the start of the century to reach 460 million tonnes, and it could triple by 2060 if nothing is done. Only nine percent is currently recycled.
Microplastics have been found everywhere from clouds to the deepest sea trenches, as well as throughout the human body.
The effects of plastics on human health remain poorly understood, but there is growing concern among scientists.
Plastic also contributes to global warming, accounting for 3.4 percent of global emissions in 2019, according to the Organization for Economic Cooperation and Development .