The demonstrations were organized by an alliance of campaigning groups set up last year who have set a goal of monthly demonstrations on the issue.
Tens of thousands of people marched across France and in Belgium Sunday to protest the lack of state action to halt climate change.
Organizers said more than 80,000 people demonstrated in French towns and cities, while police in Brussels said 70,000 turned out in the Belgian capital.
An online petition they set up on the issue — at laffairedusiecle.net — has already gathered more than 2.1 million signatures and organizers want to hit three million
"Onions, not concrete," "Less consumption, more butterflies" were among the messages on placards at a demonstration in central Paris. "It's a movement that is taking root and diversifying," Francois Dubreuil of the United for the Climate collective told France Inter radio.
As well as individual campaigners, many families attended the marches, AFP journalists noted.
"I'm here to try to slow down climate warming and to ensure that my daughter and my baby have the best planet possible," said Charlotte, from the eastern city of Strasbourg. "There's still time," she added, as she marched with her 15-month baby, while pregnant with her second child.
Gathered at the French capital's Place de la Republique square, the demonstrators, many of whom donned yellow high-visibility vests in support of the "yellow vest" movement, carried signs in support of the environmental movement.
The event took place on the 11th weekend of the anti-government "yellow vest" movement, which began in November 2018 over a carbon tax proposed by French President Emmanuel Macron's government. The tax proposal has since been scrapped.
Demonstrators said the rally established a link between climate change associations and the "yellow vests," adding that the government should penalize companies that are considered "big polluters" before imposing taxes that take a toll on the working class.
Many said France should set an example on an international level in the fight against climate change.
In Brussels, police estimated that around 70,000 people marched from the city's Gare du Nord to the European Parliament.
People from across Belgium braved cold, rainy conditions to attend the march in the capital, and extra trains had to be laid on to bring them in, the Belgian news agency reported.
So many people were arriving for the Brussels climate march Sunday that it had to start half an hour early
Sunday's turnout was larger than the 65,000 who attended the last such march in Brussels on Dec. 2.
On Thursday, a march by school and university students brought 35,000 people out on to the streets of the Belgian capital. "All disaster movies start with the government ignoring a scientist" read one placard at the Brussels demonstration.