At a rally in Chicago, Sue Meyers, a retired teacher from Frankfurt, Ill., said it was important to tell skeptics on climate change that “nowhere else in the world do people think like this.”

“The problems need to be addressed, and to deny there is a problem is even worse,” she added.

In Los Angeles, protesters gathered near the port, where the oil refiner Tesoro wants to expand its operation. “A lot more people are becoming engaged because they realize they have to,” said Kaya Foster, an environmental educator and activist from Santa Monica.

Around the country, the demonstrators’ list of grievances was long. Since taking office, Mr. Trump has appointed one of the chief antagonists of the Environmental Protection Agency, Scott Pruitt, as its administrator and proposed slashing its budget by nearly a third, more than any other federal agency. He has signed several executive orders aimed at rolling back President Barack Obama’s Clean Power Plan, a set of regulations intended to close heavily polluting coal-fired power plants, and restrictions on vehicle emissions, among others.

This past week, Mr. Trump signed orders intended to initiate reviews aimed at opening certain protected lands and waters to drilling, mining and logging. His advisers were still debating whether the United States would remain in the landmark Paris climate accord. And on Friday, the Environmental Protection Agency announced that it had taken down several agency web pages that contained climate data and other scientific information relating to climate change.

Sweltering temperatures that threatened to break a heat record in Washington on Saturday added a poetic flourish to the demonstrators’ argument.

Dire though the warnings were, the march was not without levity. One group wheeled a large “Trojan Oil Drum” that warned: “Climate Activists Inside.” White full-body polar bear suits dotted the crowd, their owners dripping in sweat underneath as they posed for seemingly endless photographs. Others hoisted miniature wind turbines, which twisted in the wind.