WASHINGTON — A federal appeals court on Friday upheld 65-year-old limits on protesters’ First Amendment rights to gather and wave signs on the grand plaza in front of the Supreme Court, reversing a district court ruling that had found the restrictions “plainly unconstitutional.”

A three-judge panel of the United States Court of Appeals for the District of Columbia Circuit ruled unanimously that restricting demonstrators to the wide sidewalks alongside the plaza was a reasonable limit to preserve the court’s decorum.

The ruling came in a case involving a man who was arrested in 2011 for protesting violence against minority groups on the court’s marble plaza. The appeals panel held that the plaza, with its tiered staircases leading to the pillared building’s formidable bronze doors, amounts to a “nonpublic forum” like the courthouse itself, a place not open to “expressive activity by the public.”

“The government retains substantially greater leeway to limit expressive conduct in such an area and to preserve the property for its intended purposes: here, as the actual and symbolic entryway to the nation’s highest court and the judicial business conducted within it,” the court wrote.