It is a simple gesture, swaddled in outrage and long-endured grief, that gained powerful currency through the protest against police brutality and racial injustice led by quarterback Colin Kaepernick on the fields of the National Football League.

Taking a knee.

Across the nation these last hard, uncertain days, demonstrators have turned to the gesture on city streets. At a nighttime march in Minneapolis on Wednesday, a crowd of 400 knelt for nearly five somber minutes. On the same day, George Floyd’s son, Quincy Mason, walked through a crowd at the site where a white police officer had pinned his father to the ground by a knee to the neck. There, before a makeshift memorial, Mason dropped to a knee.

The gesture has even been made sporadically by law enforcement officers, members of the National Guard and by prominent politicians as an act of solidarity or effort to pacify.

In New York, an N.Y.P.D. commander knelt with activists outside Washington Square Park. In Portland, Ore., police in riot gear knelt before cheering demonstrators, some of whom responded by walking toward the officers to shake their hands. Mayor Eric Garcetti of Los Angeles walked amid a demonstration and knelt. And the presumptive Democratic presidential nominee, Joseph R. Biden Jr., took a knee at a campaign visit to a black church in Delaware.

Kaepernick has not played in the N.F.L. since Jan. 1, 2017, his career cut short when no team would sign him following a season of player protest he led with the help of a teammate, Eric Reid.

But his kneeling objection during the playing of the national anthem has boomeranged through the choppy slipstream of the American consciousness, and is again at the center of a turbulent moment with newfound force, and for the N.F.L., renewed debate.

“It’s a powerful, peaceful way to say you’re not OK with what’s been happening,” said Hibes Galeano, 32, a Latina who attended a protest in Minneapolis this week. Others who knelt spoke of Kaepernick with reverence. “He did what a lot of other athletes wouldn’t have done,” said Dorien Harris, a black, 19-year-old marcher who wore a face mask inscribed with the words “I Can’t Breathe” as he knelt.

“It took a lot of guts for him to do that, a lot of heart,” he added. “He knows what the community needs. It needs that strength. He was saying to stand up for what you believe in, no matter your position.”

While some demonstrators say they have had Kaepernick and his campaign in mind when kneeling, the gesture is also — intended or not — an echo to the manner of Floyd’s death.

“Kneeling is both an act of defiance and resistance, but also of reverence, of mourning, but also honoring lives lost,” said Chad Williams, the chairman of the Department of African and Afro-American Studies at Brandeis University. “It is also simple and clear. Its simplicity gave it symbolic power, and as we see now, its power persists.”

So does the controversy surrounding it.

Starting in 2016, despite Kaepernick’s explanation that his kneeling during the national anthem was a call to end racial injustice and police brutality toward people of color, a backlash fomented, spurred largely by President Trump, who tried to recast Kaepernick and the predominantly African-American group of players who followed his lead as unpatriotic. That viewpoint persists, expressed this week by New Orleans Saints quarterback Drew Brees, who publicly apologized after saying in an interview that he views taking a knee during the anthem as an insult to the country.

“I will never agree with anybody disrespecting the flag of the United States of America or our country,” Brees said, linking such defiance to condemnation of the military.

On Friday, Trump tweeted that Brees should have not walked back his comments, saying in all caps that there should be “NO KNEELING!\" during a display of patriotism.

Taking a knee might be a simple gesture, but the fraught, contentious opinions about it are a mirror into the complexity of race in America.

Consider its N.F.L. origin story.

Kaepernick and Reid came up with the idea after consulting a former Green Beret, Nate Boyer, who had served in Iraq and Afghanistan before playing college football at Texas and then getting a tryout with the Seattle Seahawks. “Colin straight up asked me what I thought he should do,” said Boyer, speaking recently over the phone from Oregon.

Boyer said he did some research and came across a photograph of Martin Luther King Jr. kneeling in prayer and protest in Selma, Ala. during the 1960s. Boyer also remembered taking a knee at Arlington National Cemetery, in reverence of fallen friends.

“If you’re not going to stand,” Boyer told Kaepernick and Reid, as they sat in a hotel lobby hours before the 49ers’ final preseason game, against the San Diego Chargers. “I’d say your only other option is to take a knee.”

Boyer said he would never do such a thing during the anthem. But he had fought for the right of free expression, and though he said he was apolitical, he empathized with the drive to end racism and police brutality.

At the game that evening, he stood next to Kaepernick as he knelt, and felt the sting of an angry, booing crowd rain onto the field. “Maybe that was my little taste of what it is like to be black. It helped me understand,” he said.

The players’ kneeling reached a peak in the 2017 season — when Trump demanded that team owners “Get that son of a bitch off the field right now!” for kneeling — but has since largely petered out.

In early 2019, the NFL handed over a payout believed to be roughly $6 million to settle a legal fight with Kaepernick and Reid, who argued they had been denied jobs because of their actions during the national anthem.

The league agreed to donate millions of dollars to community groups and causes chosen by players. It joined with Jay-Z, the hip-hop empresario, to consult on entertainment and contribute to the league’s activism campaign, Inspire Change. It also updated a policy, so far not enforced, requiring players to stand for the national anthem or remain in the locker room while it is played.

Within a week of Floyd’s death, kneeling became a common gesture. And its complexity carries on.

The way it has been adopted by members of law enforcement and politicians, for example, is best viewed with an eye that is both skeptical and hopeful, said Mark Anthony Neal, chairman of the African and African-American Studies Department at Duke.

“It’s an important gesture, showing maybe they get it now,’’ he said. “But if those same officers and politicians are not willing to hold their own accountable going forward, or look at their own actions and examine them closely, this is at best empty rhetoric.”

Kaepernick has remained publicly silent aside from recent postings about the protest on social media.

His latest on Twitter? A retweet that sarcastically jabs at Brees and shows a 2017 photo of the Saints quarterback alongside protesting teammates, taking a knee.

Kim Barker, Dionne Searcey, John Eligon, Ken Belson and Matt Furber contributed reporting. Jack Begg contributed research.