The crowd of protesters that packed Foley Square in downtown Manhattan on Thursday was as raucous as any demonstration in a politically vociferous city. It was, however, decidedly more wonky and dapper.

About 1,500 demonstrators, many of them lawyers sporting business attire, jammed the plaza outside Manhattan’s federal courthouse as part of the National Law Day of Action, chanting in favor of the rule of law and hoisting pocket Constitutions to the sky.

It was one of around 50 similar actions around the nation on Thursday, led by lawyers who say President Trump is threatening the foundation of America’s legal system.

“The rule of law protects us all. Without it we will surely fall,” the crowd chanted.

In his second term, Mr. Trump has aimed to hobble elite law firms, threatened to impeach judges and ignored their orders. For many inside the legal profession, his actions have presented an unpalatable choice between compromising their values by staying silent and facing professional risk by speaking out.

Now, a growing number of lawyers see a moral imperative in choosing the latter option. They believe Mr. Trump’s crusade is threatening not just their livelihoods, but a system to which they have devoted their lives. On Thursday, thousands of lawyers were expected to protest at federal courthouses in New York, Chicago and San Francisco — roughly 50 cities in all.

In interviews, attendees of the event in New York pointed to a range of actions, including Mr. Trump’s targeting of law firms he viewed as hostile, wrongful deportations and the arrest of a Wisconsin judge on charges of obstructing immigration enforcement.