Presidential candidates, civil rights leaders, labor activists and thousands of citizens were coming together to honor King for his devotion to racial equality and economic justice.
King was cut down on the balcony of the Lorraine Motel on April 4, 1968, while helping organize a strike by Memphis sanitation workers, then some of the poorest of the city's working poor.
Members of the American Federation of State, County and Municipal Employees, which represented the workers then and now, marched Friday from their downtown headquarters to the motel.
A line of several hundred people carrying umbrellas in a steady rain set off on the mile-long route.
"Dr. King was like Moses," said Leslie Moore, a 61-year-old sanitation worker who began working for the city in 1968. "God gave Moses the assignment to lead the children of Israel across the Red Sea. He sent Dr. King here to lead us to a better way."
As the Rev. C.T. Vivian, a former King associate, said earlier: "Here was a man who understood nonviolence at a depth that I had never known before."
Presidential candidates Hillary Rodham Clinton and John McCain were scheduled to take part in later anniversary day events that were to include an afternoon "recommitment march" and the laying of wreaths at the motel. Sen. Barack Obama will be campaigning in Indiana.
"The whole nation flinched" when King was killed, said writer Cynthia Griggs Fleming, one of the many historians, commentators and activists in town for panel discussions and lectures on King's legacy.
King advised his followers to keep working for equal rights for all citizens, "to keep on moving," no matter what obstacles they faced, Fleming said in a talk Thursday at a Memphis church.
"Don't be so consumed by the pain that you don't hear the message," she said.
King's son, Martin Luther King III, wrote in an opinion piece published in The Atlanta Journal-Constitution on Thursday that the nation is still plagued by poverty. He urged presidential candidates to vow to appoint a Cabinet-level officer who would help the poor.
"We are not doing anywhere near enough," he said Friday during an interview with his sister, Bernice, on the "Today" show.
The National Civil Rights Museum opened in 1991 at the former motel, which now holds most of the exhibits tracing the history of America's struggle for equal rights. The museum also encompasses the flophouse across the street from which confessed killer James Earl Ray admitted firing the fatal shot. Ray died in prison in 1998.
King was a champion of nonviolent protest for social change, and his writings and speeches still stir older followers and new ones alike, said Vivian, who helped organize lunch-counter sit-ins in Nashville in 1960 and rode on a "freedom bus" through Mississippi.
"The world still listens to Martin," he said. "There are people who didn't reach for him then who reach for him now. They want to know this man. What did he say? What did he think?"
Other tributes were being held around the country. In Congress, House and Senate leaders and lawmakers who once worked with the civil rights leader marked the anniversary with a tribute Thursday in the Capitol's Statuary Hall.
"Because of the leadership of this man we rose up out of fear and became willing to put our bodies on the line," said Rep. John Lewis, D-Ga., a companion of King in the civil rights struggles of the 1960s.
In Indianapolis, Ethel Kennedy was scheduled to make brief remarks during a ceremony Friday evening at what is now Martin Luther King Jr. Park. Her late husband Robert Kennedy gave a passionate speech there the night of King's assassination that was credited with quelling violence in the city.
In Atlanta, the Martin Luther King Jr. Historic Site was commemorating the anniversary with the opening Friday of a special exhibit chronicling the final days and hours before King's death, as well as his funeral procession through his hometown five days later.
The centerpiece of the exhibit is the wagon that was drawn by two mules as it carried King's casket from his funeral at Ebenezer Baptist Church to Morehouse College, his alma mater.
Memphis has also been in the news lately because of the success of the Memphis Tigers, who play UCLA in the Final Four on Saturday. Coach John Calipari had copies of King's "I Have a Dream" speech for his players to read after practice Wednesday, along with a King biography, and the Rev. Jesse Jackson met the team for a personal history lesson.