Climate activists gathered 650,000 cigarette butts and piled them up in the heart of Portugal's capital Lisbon on Sunday to raise awareness about the often-overlooked form of pollution.

They collected enough stubs to fill 40 plastic buckets, then heaped them into a mound for the German climate campaigner behind the stunt to climb on.

Andreas Noe wore a breathing apparatus to protect him from the toxins in the non-biodegradable butts.

"We asked everyone in Portugal to take part in this community project to raise awareness about plastic pollution, because plastic is hidden in cigarette butts and many people don't know it," the 34-year-old said.

He said the simple cigarette butt was a "beautiful example of how someone can start to take action" against littering, ocean pollution and ultimately the climate crisis.

Two years ago, the activist and surfing enthusiast collected about a million cigarette butts in two months.

"We want to put an end to this problem by encouraging people to throw their butts in the bin or pocket ashtrays -- anywhere but on the ground," said David Figueira, who took part in the project.