LAKE BUENA VISTA, Fla. — George Hill is hardly the biggest star in professional basketball. But he was the one who took the lead when a handful of players on the Milwaukee Bucks began talking about the police shooting of Jacob Blake a few days earlier in Wisconsin.

The players, led by Hill, implored their teammates not to play in their playoff game on Wednesday, believing they had a responsibility to make a statement about the how the police treat Black people.

What they envisioned — a one game, on-the-fly protest — instead inspired one of the broadest political statements across sports leagues that the United States had ever seen: walkouts involving hundreds of athletes in professional men’s and women’s basketball, baseball and soccer, as well as one of the world’s biggest tennis stars.

LeBron James, basketball’s most famous athlete, said on Twitter that change “happens with action and needs to happen NOW!” President Trump, who had previously attacked the league and had publicly sparred with James, who plays for the Los Angeles Lakers, said people were “a little tired of the N.B.A.”

By Thursday afternoon, the N.B.A. players had pledged to return to play, according to three people who were part of the discussions and spoke on condition of anonymity because final details of a comeback had not been worked out.

“We are hopeful to resume games either Friday or Saturday,” a league spokesman, Mike Bass, said in a statement.

But the players’ message was still echoing — within and well beyond the world of sports, perhaps to greater effect than ever before.

More baseball, hockey and basketball games were called off on Thursday, along with football practices and other events as athletes urged greater focus on conversations about racism and police brutality.

Basketball players, especially women, have been at the forefront of discussions and demonstrations about social injustice for years. That has morphed amid the police killings of George Floyd and Breonna Taylor, and the shooting of Blake, a Black man who was shot in the back multiple times by the police in Kenosha, Wis., as he tried to enter his vehicle.

Following their walkout, Bucks players, led by Hill and his teammate Sterling Brown, called for elected officials in Wisconsin to take concrete steps to hold the police officers accountable for how they treated Blake.

“For this to occur, it’s imperative for the Wisconsin state Legislature to reconvene after months of inaction and take up meaningful measures to address issues of police accountability, brutality and criminal justice reform,” Hill said.

The walkouts drew the attention of several prominent political figures. Among the first to weigh in was former President Barack Obama, who has personal relationships with several N.B.A. players. He praised the Bucks in a tweet on Wednesday “for standing up for what they believe in.”

The reaction from the White House was much more critical.

Along with Trump’s comments, Jared Kushner, the president’s son-in-law and one of his senior advisers, said on CNBC, “I think that the N.B.A. players are very fortunate that they have the financial position where they’re able to take a night off from work without having to have the consequences to themselves financially.”

The players’ action, as impactful as it has been, has also come with some of its own challenges. At a private players’ meeting hours after the walkout, some players expressed frustration with the Bucks for surprising the union, their opponents and the league with their protest, according to two people who attended the meeting but were not authorized to discuss the details publicly.

Then, after the protest spread and inspired some players to reconsider playing at all this season, the Bucks quickly backed resuming play, rankling some rival players, including James, according to the people.

Several N.B.A. and W.N.B.A. players have increased their social justice efforts in recent months. James and other top athletes formed More Than a Vote to protect voting rights and reach out to Black voters. Stephen Curry, the Golden State Warriors guard, appeared in a video at the Democratic National Convention in support of Joseph R. Biden Jr., the party’s presidential nominee. Renee Montgomery, who plays for the W.N.B.A.’s Atlanta Dream, skipped the season altogether to focus on social justice efforts.

Recently, the entire Atlanta team and others in the league publicly endorsed an opponent of Senator Kelly Loeffler, a Republican from Georgia who is a co-owner of the Dream, because she criticized the Black Lives Matter movement and the W.N.B.A. players’ social activism.

Earlier this month, the N.B.A.’s owners, many of them billionaires, said they would donate $300 million over 10 years — roughly $1 million per year for every team — to a fund “dedicated to creating greater economic empowerment in the Black community,” according to a news release. In a Thursday afternoon video conference, N.B.A. players — the Lakers’ James among them — asked for a greater commitment from the owners, with the Charlotte Hornets’ owner Michael Jordan joining the call as much as an advocate for the players as an ownership peer.

Tilman Fertitta, the owner of the Houston Rockets, said on CNBC that he did not think that the stoppage was directed at the league’s owners.

“I think they just needed a pause,” Fertitta said Thursday. “They’ve been playing every other day. These are our partners, OK? We’re 50-50 partners. And if we do well, they do well. And if the league doesn’t do well, none of us do well. And they realize that.”

With basketball games called off on Thursday, the prior day’s insurgency led professional and college teams in other sports to hurriedly scratch their plans, too.

In baseball, at least seven games were postponed. The Mets and Marlins stood on the field at game time for 42 seconds of silence; 42 was the number worn by Jackie Robinson, who broke baseball’s color barrier in 1947. The players then retreated to their clubhouses, leaving behind a “Black Lives Matter” T-shirt over home plate.

Ron Rivera, the coach of the professional football team in Washington, a franchise with a troubled history on race, said the day would be reserved for “reflection instead” of football. The New York Jets, the Indianapolis Colts and the Green Bay Packers were among the teams that also canceled practices.

Troy Vincent, the N.F.L.’s head of football operations and the highest ranking Black person at the league office, spoke tearfully on ESPN Radio about his fears for his three sons and his support for the athletes who sat out games.

“I’ve got a 22-year-old and a 20-year-old and a 15-year-old that I’m trying to prevent from being hunted, and their teachable moment and I’m trusting my Lord, trusting him,” Vincent said. “I’m just — I’m proud of what the guys and the women are doing.”

Boston College, Kentucky and South Florida abandoned plans for football practices on Thursday, unsurprising moves toward the end of an unusually vibrant off-season of activism by college athletes.

“What happened to Jacob Blake is history repeating itself,” Max Richardson, a linebacker at Boston College, wrote on Twitter. “These countless tragedies are reoccurring. There can be no more ignorance. Changes WILL come.”

Playoff hockey games that were scheduled for Thursday and Friday were also postponed.

It was not always clear how long the various teams and players would keep their sports activities on pause.

Four baseball teams that had protested on Wednesday resumed play on Thursday, and Naomi Osaka, a Black woman who has won two Grand Slam titles, said she would play in the Western & Southern Open’s semifinals on Friday. Osaka had planned to quit the tournament because there were “more important matters at hand,” but she relented after tennis officials agreed to delay matches by a day.

By the end of Thursday, some in sports said they were already turning to political action beyond demonstrations. Mike Krzyzewski, the men’s basketball coach at Duke, told a rally of athletes that his players would register to vote later in the day.

“This thing can be won, and your generation is the generation that’s going to do it,” he said without endorsing any particular candidate. “I grew up a long time ago in the ’60s. I thought it was headed in the right direction. Damn, I was wrong. I want to be right.”

Marc Stein reported from Lake Buena Vista, Sopan Deb from New York and Alan Blinder from Atlanta. Reporting was contributed by Ken Belson, Gillian R. Brassil, Christopher Clarey, Matthew Futterman, Shauntel Lowe, Ben Rothenberg and David Waldstein.