Brussels and Paris have been the site of strong protests demanding more action on climate change.
At least 70,000 people have braved cold and rain in Brussels to demand the Belgian government and the European Union increase efforts to fight climate change.
The event was described as Belgium's biggest climate march ever, with trains so clogged that thousands of people didn't make the march in time.
"Young people have set a good example," protester Henny Claassen said amid banners urging better renewable energy use and improved air quality.
"This is for our children, for our grandchildren, and to send a message to politicians."
The march ended at the headquarters of the European Union. The 28-nation bloc has been leading global efforts to counter climate change but still came in for the protesters' criticism.
"Society as a whole could do a lot more because they're saying 'Yes, we're doing a lot,' but they're doing not that much. They could do a lot more," demonstrator Pieter Van Der Donckt said.
Citizen activism on climate change Sunday was not limited to Belgium.
Thousands of people made human chains or held other climate events around France.
In Paris, there was a debate inspired by a recent petition for legal action to force the government to set more ambitious goals for reducing carbon emissions that create global warming.
President Emmanuel Macron sees himself as a climate crusader, but suffered a serious setback when fuel tax increases meant to help wean France off fossil fuels backfired dramatically, unleashing the country's yellow vest protests now in their third month.