John Salter Jr., later known as John Hunter Gray, sits in the foreground at a Woolworth’s lunch counter in Jackson, Miss., where he and other demonstrators were assaulted on May 28, 1963. (Fred Blackwell/AP)

When John Salter Jr. arrived in Mississippi in 1961, he was 27 and had already served in the Army, worked as a labor organizer in Arizona, received a master’s degree in sociology and taught at a college in Wisconsin. He was taking a new job as a professor at Tougaloo College, a historically black institution a few miles outside the state capital of Jackson.

Mr. Salter did much more than teach at Tougaloo. He became a close associate of Medgar Evers, the NAACP’s Mississippi field secretary, and helped organize what became known as the Jackson Movement.

Mr. Salter, who later changed his name to John Hunter Gray to honor his American Indian heritage, was perceived as white at the time.

Mr. Salter — as he was known throughout his career as an activist and academic — died Jan. 7 at his home in Pocatello, Idaho, at age 84. He had recovered from systemic lupus, said a son, John Salter III, who could not cite a specific cause of death.

In Mississippi, the home of Mr. Salter and his wife became an informal headquarters for civil rights activists.

“We decided to go south because things were happening,” he told filmmaker Loki Mulholland for his documentary “An Ordinary Hero,” about his mother, Joan Trumpauer Mulholland, who was one of Mr. Salter’s students at Tougaloo.

“When history calls, you usually get, I would say, maybe one chance to do something,” Mr. Salter added, “and then if you reach out for it, you ride off on the wings of destiny.”

At Tougaloo, he organized an NAACP youth council, taught tactics of nonviolence and published a mimeographed newsletter, “North Jackson Action.” He conducted a study of poverty in Mississippi for the NAACP and testified before the U.S. Commission on Civil Rights.

He was also under surveillance by the FBI, which compiled a file of several thousand pages about him. Police officials often tailed Mr. Salter as he walked on the streets or drove on local roads. He once found the lug nuts on his car wheels loosened.

Nonetheless, Mr. Salter became “the Jackson Movement’s radical heart,” as M.J. O’Brien wrote in his 2013 history, “We Shall Not Be Moved: The Jackson Woolworth’s Sit-In and the Movement It Inspired.”

After a series of commercial boycotts and protests, the Jackson Movement burst into the nation’s consciousness with the dramatic events of May 28, 1963.

That day, Evers called for Mississippi’s first sit-in. Students from Tougaloo and other activists planned to enter a Woolworth’s department store, sit down at a segregated lunch counter and ask for service.

An all-white crowd of several hundred, many of them young men from a local high school, quickly gathered and began to taunt the protesters, pulling the women off their stools. The male demonstrators were physically attacked.

Mr. Salter soon joined the protesters at the lunch counter. Reporters and a photographer for the Jackson Daily News, Fred Blackwell, captured what happened next.

A former Jackson police officer, Benny Oliver — fired for his violent tendencies — punched black demonstrator Memphis Norman in the head, knocking him down.

“As Mr. Norman lay on the floor,” a New York Times account noted, “Mr. Oliver began kicking him in the face until blood spurted from his mouth and nose.”

Police officers, who otherwise stood idly by, placed both Oliver and Norman under arrest.

In Blackwell’s famous photograph of the incident, Mr. Salter sits in the left foreground at the counter alongside two Tougaloo students, Joan Trumpauer — one of the college’s two white undergraduates — and Anne Moody.

“Annie and I were pulled off our stools,” Trumpauer, now Joan Mulholland, said in an interview. “I didn’t get any bad blows, but I had every condiment on the counter poured on my head.”

Mr. Salter was struck with brass knuckles and slashed in the back of the head with the edge of a broken glass sugar container. A watery pepper solution was thrown in his eyes. His cuts and abrasions were filled with salt and pepper. Blood dripped onto his shirt and mixed with the ketchup, mustard and sugar that had been dumped on his head.

Cigarettes were stubbed out on the back of Mr. Salter’s neck, leaving scars that were visible for the rest of his life.

“We knew we were going to die,” Joan Mulholland said. “My spirit had left my body and was hovering somewhere above, protecting me.”

The ordeal lasted for three hours. The police stepped in only when the mob began to loot and damage the store.

The demonstrators were escorted out only after the president of Tougaloo College was able to organize a caravan of cars to take them to safety.

In the next few days, further demonstrations were held in Jackson. Police tactics grew tougher, as Mr. Salter and other protesters were arrested, hauled away in garbage trucks and held in cattle pens at the local fairgrounds.

“In some instances,” Mr. Salter recalled, “police urinated in water buckets, threw the food on the ground and said, ‘Eat, dogs, eat.’ ”

On June 11, two weeks after the lunch-counter protest, President John F. Kennedy went on national television to call for a comprehensive national civil rights bill. That night, Evers was assassinated at his home.

The next day, Mr. Salter was arrested during a protest march and beaten until he “woke up in a pool of muddy blood.” He was taken to the fairgrounds, where other demonstrators were being held.

“I stepped out of the paddy wagon and held my fist up,” he said. “You know in a nice early ’60s stance. Probably one of the first times that had been done, and was immediately cheered by a whole throng of people.”

After a suspicious car accident in which Mr. Salter and a chaplain were seriously injured, he left Mississippi and continued his civil rights activities in North Carolina and elsewhere. A year later, President Lyndon B. Johnson signed the Civil Rights Act of 1964 into law.

John Randall Salter Jr. was born Feb. 14, 1934, in Chicago. He grew up mostly in Flagstaff, Ariz., where his father, an American Indian descended from several tribes in the Northeast, was an artist and college professor. His mother was a teacher.

After serving in the Army and helping organize labor unions at mining camps, Mr. Salter graduated in 1958 from Arizona State University, from which he received a master’s degree in sociology in 1960.

He alternated between teaching and activism, moving from Vermont to the state of Washington, from Rochester, N.Y., to Chicago, where he worked with street gangs. He taught at the University of Iowa, at a college for Navajo Indians (now Diné College) in Arizona and from 1980 to 1994, at the University of North Dakota, where he directed the Department of American Indian Studies.

Mr. Salter, who settled in Idaho in 1997, legally changed his name in homage to his ancestors; Gray had been his father’s last name before he was adopted by a white family.

His wife of 54 years, the former Eldri Johanson, died in 2015. Survivors include four children, Maria Salter and Josephine Evans, both of Pocatello, and John Salter III and Peter Salter, both of Lincoln, Neb.; a brother; 12 grandchildren; and two great-grandchildren.

In 1979, Mr. Salter published a book about his civil rights experiences, “Jackson, Mississippi: An American Chronicle of Struggle and Schism.”

“Nobody who was part of it can forget it,” he told filmmaker Mulholland. “You don’t have to remember every detail, just remember that you know there was a time when a pretty hideous situation existed and there were people who set out to change it.”