A man whose skull was fractured after he was shoved to the ground by police officers at a protest in Buffalo last year has filed a lawsuit against the city and members of its police force.

The man, Martin Gugino, 76, claims in the lawsuit that he was “forcibly assaulted” by the police at the protest, and that other officers walked by as he “lay unconscious on the sidewalk, blood pouring from his fractured skull.” The suit, filed on Monday in the U.S. District Court for the Western District of New York, also alleges that the defendants tried to conceal the assault.

The filing, which claims that Mr. Gugino’s constitutional rights were violated, was the latest development in an episode that was captured on video and widely shared on social media last year. It added outrage to a period of protests over police violence after the deaths of George Floyd in Minneapolis and Breonna Taylor in Louisville, Ky.

One video captured by a local radio station shows two officers pushing Mr. Gugino, causing him to stagger backward and land hard on the sidewalk. Blood can be seen pooling beneath his head. The Buffalo Police Department said at the time that he “tripped and fell” during a “skirmish” after an 8 p.m. curfew.

Richard P. Weisbeck, one of the lawyers who filed the lawsuit, said in a statement on Monday that Mr. Gugino had a constitutionally protected right to be at the protest in Niagara Square, Buffalo’s center of government. Instead, Mr. Gugino “became the victim of police brutality at the very moment he was peaceably and constitutionally protesting against police brutality,” Mr. Weisbeck said.

The lawsuit names Officers Robert McCabe, Aaron Torgalski and John Losi as defendants, as well as Police Commissioner Byron C. Lockwood and the deputy commissioner, Joseph Gramaglia.

Mayor Byron W. Brown, who ordered the curfew and said on his Facebook page on June 4 that Mr. Gugino had been involved in a “physical altercation” when he was “knocked down,” was also named as a defendant.

Michael DeGeorge, a spokesman for the city of Buffalo, said in an email on Tuesday that the city did not comment on pending litigation.

Officers Torgalski and McCabe were charged in June with felony assault for what the Erie County district attorney, John J. Flynn, said were actions that “crossed the line” and that “violated the law.” Both officers, members of the Police Department’s Emergency Response Team, pleaded not guilty and were released. They were suspended without pay.

This month, a grand jury declined to indict the officers.

On June 4, Mr. Gugino, an activist and a member of the Western New York Peace Center, joined protesters in Niagara Square, the lawsuit says. After days of protests in the city, a curfew was put in place for June 2 to June 8, from 8 p.m. to 5 a.m.

The lawsuit says that Mr. Gugino arrived at the square and went to City Hall, where approximately 15 protesters were on the sidewalk and steps, at about 7:28 p.m.

According to the lawsuit, Mr. Gugino became alarmed when a line of Emergency Response Team officers in riot gear marched toward him and two other protesters on the steps, and he stood up to talk to them shortly after 8 p.m. The officers said to “push him, push him,” the lawsuit claims.

Officer Losi shoved Officers McCabe and Torgalski toward Mr. Gugino, and they “unlawfully assaulted” him “by forcibly pushing him to the ground, where he laid incapacitated and seriously injured with blood pooling from his ear,” the lawsuit says.

“The sound of Martin Gugino’s skull fracturing could be heard almost instantly and was captured on video by nearby reporters,” the lawsuit continues. It alleges that Officer McCabe tried to check on Mr. Gugino but that Officer Losi stopped him.

More than a dozen Buffalo officers walked by as he lay bleeding on the sidewalk in front of City Hall, the lawsuit says. Mr. Gugino was treated in the hospital for a concussion and a fractured skull, and released after nearly four weeks, according to the lawsuit, which is seeking a jury trial and damages.

One of Mr. Gugino’s lawyers, Melissa D. Wischerath, said in an email on Tuesday that Mr. Gugino experienced long-term complications from the fracture and a brain hemorrhage, including mobility and memory impairments, hearing loss and complications in his right ear.