A federal district judge declared unconstitutional Wednesday a law that bans organized protests and signs on the marble plaza in front of the Supreme Court.

Judge Beryl A. Howell called the legislation passed by Congress in 1949 “unreasonable” and “substantially overbroad.”

“It cannot possibly be consistent with the First Amendment for the government to so broadly prohibit expression in virtually any form in front of a courthouse, even the Supreme Court,” Howell wrote in a 68-page opinion.

The law that Howell threw out says this: “It is unlawful to parade, stand, or move in processions or assemblages in the Supreme Court Building or grounds, or to display in the Building and grounds a flag, banner, or device designed or adapted to bring into public notice a party, organization, or movement.”

Her order, however, appeared to refer only to the Supreme Court plaza, rather than the building or its other grounds.

It was on the plaza that Harold Hodge, 46, of southern Maryland was arrested in January 2011. He was wearing around his neck a 3-by-2-foot sign that said, “The U.S. Gov. Allows Police To Illegally Murder And Brutalize African Americans And Hispanic People.”

The charge against him was dismissed after he agreed to stay away from the Supreme Court building and grounds for six months.

But Hodge challenged the law with the help of the Rutherford Institute, which celebrated the decision Wednesday.

“Judge Howell’s frank, no-holds-barred ruling affirming the Supreme Court plaza as a free speech zone throws a lifeline to the First Amendment at a time when government officials are doing their best to censor, silence and restrict free speech activities,” Charles A. Whitehead, the institute’s founder, said in a statement.

The suit named as defendants Supreme Court Marshal Pamela Talkin and Ronald Machen Jr., the U.S. attorney for the District of Columbia. Spokespeople for both said their offices are studying the decision and had no comment about whether it would be appealed.

Any visitor to the court on the day an important case is argued would be hard-pressed to describe it as a protest-free zone. That’s because the Supreme Court in 1983 said the law did not apply to the public sidewalks along First Street, which are frequently filled with singing and chanting protesters.

But that ruling did not directly address protests on the oval-shaped marble plaza in front of the court, which is eight steps up from the sidewalk and has two large fountains and long, curving benches.

The government argued that the law is necessary to protect public access to the court, maintain proper order and decorum, and preserve the image of the court as a body not swayed by external influence.

But Howell — who was nominated by President Obama and joined the court in late 2010 — said there are ways to accomplish those goals without a law so broad that it could be used to ban conversations between a small group of people on the plaza or groups wearing the same school T-shirts.

She noted that a D.C. law allows law enforcement officials to prevent protesters from blocking the entrance to buildings.