Rev. Al Sharpton called for national policing legislation akin to the Civil Rights Act this morning at the kickoff his National Action Network’s annual convention, just after the arrest of a white South Carolina police officer for murder in the shooting of an unarmed black man.

“There must be national policy and national law on policing,” Mr. Sharpton said. “We can’t go from state to state, we’ve got to have national law to protect people against these continued questions.”

Mr. Sharpton’s comments, coming on the heels of multiple instances of police killings of unarmed men of color around the country, were met with applause from the crowd—and from the dais, which was packed with elected officials including Mayor Bill de Blasio, Congressman Charles Rangel, Attorney General Eric Schneiderman, city Comptroller Scott Stringer and state Comptroller Thomas DiNapoli

The convention kickoff, which featured a ribbon cutting with the lawmakers, came just hours after it was announced last night that North Charleston, S.C., police officer Michael T. Slager would be charged with murder in the death of Walter Scott—who can be seen in a widely publicized video running away from Mr. Slager, while the officer shoots into the man’s back repeatedly. The video offers a markedly different story than the one Mr. Slager first offered up: that Scott had stolen his taser and left him in fear for his life.

Mr. Sharpton praised the city’s mayor and police chief for bringing the charges, but said the nation couldn’t rely on the judgement of local officials.

“We commend them, but we cannot have a justice system that hopes we have a mayor in the right city or a police chief,” he said. “We have to have one policy that is national.”

Mr. Sharpton later noted that the comparatively small town’s officials had been braver than police leaders in bigger cities. He has been vocal about his belief that New York City police Daniel Pantaleo should have been charged with a crime in the death of an unarmed black Staten Island man, Eric Garner. A grand jury declined to indict Mr. Pantaleo, spurring protests throughout the city.

That death, too, was captured in a widely published video. And though the footage did not lead to any charges, Mr. Sharpton said today the national legislation should focus on “cameras” as well as “accountability.”

He compared the fight for police reform to the civil rights struggle, noting that activists did not try to fix discrimination in individual states or cities.

“They fought for a national Civil Rights Act, a national Voting Rights Act. It’s time for this country to have national policing,” Mr. Sharpton said.

After the ribbon cutting, Mr. de Blasio—whose first year in office was dominated by an effort to reform police-community relations after Garner’s death and a subsequent City Hall feud with police union leadership—said he agreed some kind of national standard should be set.

“It’s a broad point he’s making, and I think he way he made the analogy to the Voting Rights Act is the right one. We’ve got to figure out how to create the right relationship between police and community,” Mr. de Blasio told reporters. “The vast majority of police do their job well and want to work more closely with the community. Obviously community residents want to work more closely with the police. But we have to create more of a national standard that says we all have to be on the same page.”

The relationship between Mr. de Blasio and Mr. Sharpton was fodder for his woes with police unions last year: they took umbrage when Mr. Sharpton was seated next to the mayor and Police Commissioner Bill Bratton at a City Hall round table, and dismissed Mr. Sharpton as divisive. A poll later showed voters didn’t like the approaches of either Mr. Sharpton or the union leaders and rank-and-file officers who later turned their backs on Mr. de Blasio at the funeral for two slain officers.

Today, Mr. Sharpton offered a full-throated defense his relationship with the mayor, saying it was based not on political power but on a long history of working together, citing Mr. de Blasio’s support on issues like wage increases and the silent march against stop, question and frisk before his election.

“He marched with us when other candidates wouldn’t. So don’t begrudge us for knowing somebody that we always knew, and that was there in the trenches with us,” Mr. Sharpton said, saying he had never asked for favors or back room deals—only access and policy changes. “There’s nothing in the back room we want. We want everything out front.”