Coretta Scott King, the widow of slain civil rights leader Martin Luther King Jr. who worked tirelessly to pursue his dream of equality, has died. She was 78.

Former Atlanta Mayor Andrew Young, the civil rights activist who is close to the King family, broke the news on NBC's "Today" show on Tuesday morning.

"I understand that she was asleep last night and her daughter went in to wake her up and she was not able to and so she quietly slipped away. Her spirit will remain with us just as her husband's has," he said.

In August, King suffered a minor heart attack and a debilitating stroke that impaired her ability to speak.

She died early Tuesday of respiratory failure at the Santa Monica Health Institute in the Mexican beach resort of Rosarito, where she had arrived last week, partially paralyzed and with advanced ovarian cancer.

Coretta Scott was born April 27, 1927, in Perry County, Ala.

To help her family during the Depression, young Coretta picked cotton.

She later received a bachelor of arts in music and education from Antioch College in Yellow Springs, Ohio, and then went on to study concert singing at Boston's New England Conservatory of Music, where she earned a degree in voice and violin.

It was in Boston that a friend introduced her to Martin Luther King Jr., a young Baptist minister working toward a doctorate in systematic theology at Boston University.

"She said she wanted me to meet a very promising young minister from Atlanta," King once said, adding with a laugh, "I wasn't interested in meeting a young minister at that time."

She recalled that on their first date he told her: "You know, you have everything I ever wanted in a woman. We ought to get married someday."

Eight months later, on June 18, 1953, they did.

King, who was in Atlanta when her husband was assassinated in Memphis, Tenn. in 1968, learned of the shooting in a telephone call from Rev. Jesse Jackson.

It was the call, she later wrote, "I seemed subconsciously to have been waiting for all of our lives."

Though she served as a lieutenant in her husband's civil rights movement, she became a symbol in her own right for the struggle for equality.

As she recalled in her autobiography My Life With Martin Luther King Jr., she felt she had "rededicate myself to the completion of his work."

Indeed, she was determined to keep his ideology of equality at the forefront of the nation's agenda.

"I'm more determined than ever that my husband's dream will become a reality," King said soon after his slaying.

For more than a decade, she lobbied to have her husband's birthday observed as a national holiday.

In 1983, U.S. President Ronald Reagan signed the bill into law, and the first federal holiday was celebrated in 1986.

Determined to uphold her husband's dream of a colour-blind society, she create a memorial and forum in the Martin Luther King Jr. Center for Nonviolent Social Change in Atlanta.

The centre, where the flags were lowered to half-mast Tuesday morning, contains archives with more than 2,000 King speeches and is built around the King crypt and its eternal flame.

Her steely resolve won her millions of admirers.

Rev. Jesse Jackson said she should be remembered as a "servant of the struggle" whose strength fortified her husband's will.

"Her ears were his listening posts, her voice calmed his fears, and so when we think of Dr. King's legacy, Coretta Scott King's legacy and Dr. King's are inextricably bound as authentic freedom fighters, human rights leaders who gave their best against the odds and made all of us better and better off," he said, appearing on CTV Newsnet.

Meanwhile, poet Maya Angelou said King's passing was both "bleak" and "great" because it provided the opportunity to reflect on her accomplishments.

"It's bleak because I can't -- many of us can't hear her sweet voice but it's great because she did live, and she was ours. I mean African-Americans and white Americans and Asians, Spanish-speaking -- she belonged to us and that's a great thing," she said on ABC's "Good Morning America."

Rep. John Lewis, a Democratic congressman from Georgia and civil rights leader, said her death was "a very sad hour."

"Long before she met and married Dr. King, she was an activist for peace and civil rights and for civil liberties," he told CNN.

Sen. Edward Kennedy, a Democrat from Massachusetts, called King "a driving force, not just for the civil rights movement, but for the great march toward progress."