Mrs. King, 78, suffered a stroke and a mild heart attack last August. As part of her rehabilitation, she was receiving further medical treatment at Hospital Santa Monica, a holistic health center, when she died, her sister Edythe Scott Bagley told my mother.
President Bush and the first lady were "deeply saddened" to hear about Mrs. King's death, White House spokesman Scott McClellan told reporters Tuesday.
"Mrs. King was a remarkable and courageous woman and a great civil rights leader," the president said in a statement.
"Laura and I were fortunate to have known Mrs. King, and we will always treasure the time we spent with her," Bush said. "We send our condolences and prayers to the entire King family."
"This is a very sad hour," U.S. Rep. John Lewis, a Democrat from Georgia, told CNN on Tuesday.
"She was the glue. Long before she met and married Martin Luther King Jr. she was an activist," he said.
English: Dr. Martin Luther King giving his "I Have...
Martin Luther King leaning on a lectern. Deutsch: ...
President Lyndon B. Johnson and Rev. Dr. Martin Lu...(Watch how she balanced motherhood and the movement -- 3:42)
"She would always admonish us that ... one of the ways you bring about change is, you must change yourself so that you're prepared to lead people in the direction they should go. If your emotions are as bad as those you're fighting, even if your cause is just, you disqualify yourself from being effective," the Rev. Al Sharpton told CNN on Tuesday.
The Rev. Fred Shuttlesworth, a family friend, described her as a "matriarch of the movement, a patriot of all that America stands for," in an interview with CNN affiliate WSB-TV in Atlanta.
Tuesday, the flag at the Martin Luther King Jr. Center for Nonviolent Social Change, which Mrs. King founded in Atlanta as a memorial to her husband's work and dream after his assassination, was flying at half-staff.
Mourners stopped at the center to pay their respects, many of them visibly upset. Some carried flowers.
"We appreciate the prayers and condolences from people across the country," her family said in a statement.
Funeral arrangements will be announced once plans are finalized, the statement said.
Georgia Gov. Sonny Perdue has ordered flags at all state buildings to be flown at half-staff until Mrs. King's funeral. He also has offered to have her body lie in state at the capitol building rotunda in Atlanta, the governor's spokeswoman Heather Hedrick told CNN.
The family has not yet responded to that offer, Hedrick said.
Mrs. King's last public appearance came January 14 at a Salute to Greatness dinner as part of the Martin Luther King Day celebrations in Atlanta.
She received a standing ovation and, supported on the arms of her children, waved to the crowd.
She did not speak at the event and was in a wheelchair.
It had been more than 20 years since she oversaw the first legal holiday in honor of her husband, a holiday that has come to be celebrated in some form in over 100 countries, according to her biography on The King Center Web site.
'She stood for peace'
Born in Marion, Alabama, on April 27, 1927, Coretta Scott graduated as valedictorian of her high school class and attended Antioch College in Yellow Springs, Ohio. She received a B.A. in music and education and then studied concert singing at the New England Conservatory of Music in Boston, Massachusetts. She got a degree in voice and violin, according to her biography.
While there, she met a theology student from Atlanta, Martin Luther King Jr., who was pursuing a doctorate at Boston University. They married on June 18, 1953, in her hometown of Marion.
As the young pastor began his civil rights work in Montgomery, Alabama, Coretta Scott King worked closely with him, organizing marches and sit-ins at segregated restaurants while raising their four children: Yolanda Denise, Martin Luther III, Dexter Scott and Bernice Albertine.
Mrs. King performed in "Freedom Concerts," singing and reading poetry to raise money for the Southern Christian Leadership Conference, the organization which Dr. King led as its first president.
The family endured the beating, stabbing and jailing of the civil rights leader, and their house was bombed.
When James Earl Ray killed her husband in Memphis in 1968, just prior to a planned march, Mrs. King organized his funeral, then "went to Memphis and finished the march," the Rev. Jesse Jackson said Tuesday.
"She was a staunch freedom fighter," he added.
Mrs. King turned her grief into the nurturing of her husband's legacy. The year her husband was killed, she established The King Center. A year later, she published her memoir, "My Life with Martin Luther King, Jr."
She spoke out "on behalf of racial and economic justice, women's and children's rights, gay and lesbian dignity, religious freedom, the needs of the poor and homeless, full employment, health care, educational opportunities, nuclear disarmament and ecological sanity," according to her biography.
Mrs. King and three of her children were even arrested in 1985 while protesting apartheid at the South African embassy in Washington, according to her official biography.
"I believe what Coretta Scott King would want us to do is continue this march toward progress when it comes to disability rights, women's rights, civil rights -- and not retreat from it," said Sen. Ted Kennedy, a Democrat from Massachusetts.
"She wore her grief with dignity," said the Rev. Joseph Lowery, former president of the SCLC, who worked on civil rights with Dr. King in the 1950's. "She moved quietly but forcefully into the fray. She stood for peace in the midst of turmoil."