Thousands of protesters opposing broad swaths of President Trump’s agenda took to the streets on Monday across the United States, calling Mr. Trump a “king” on Presidents’ Day for his efforts to terminate thousands of federal workers and to fire prosecutors and independent watchdogs within the federal government.

On Saturday, Mr. Trump suggested on social media that he would not heed concerns that his sweeping actions could be breaking laws, posting a riff on a phrase often attributed to Bonaparte Napoleon: “He who saves his country does not violate any law.”

“No king, no crown, we will not back down,” chanted those who gathered a few hundred feet from the U.S. Capitol on the National Mall in Washington, D.C.

Many protesters opposed to Mr. Trump’s agenda embraced symbols of patriotism, waving flags, and wearing heart-shaped earrings and beanies adorned with the U.S. flag. The members of an a cappella group sang the national anthem by the Capitol Reflecting Pool. The audience burst into applause after the final line: “And the home of the brave.”

“It’s our anthem, it’s our flag, it belongs to us,” said Shawn Morris, the president of Gay Men’s Chorus of Washington, D.C. “We wanted to take it back.”

The major group organizing the protest identified itself as the 50501 movement, a grass roots effort to push back against what it views as Mr. Trump’s second term “overreach” in reshaping of the government. Sarah Parker, the executive director of 50501’s member organization Voices of Florida, said the nationwide protests were fueled by a Reddit post that was written and commented by those who were angered from Mr. Trump’s actions and looking for ways to act upon their frustrations.