King was admitted to Piedmont Hospital on Tuesday, and the family issued a statement Wednesday saying she was resting comfortably. It expressed thanks for the "outpouring of care and support that's being sent from around the world."
Hospital spokeswoman Diana Lewis and the family wouldn't discuss her illness, but the Rev. Joseph Lowery said King had suffered a stroke and was having trouble speaking.
King had canceled some recent public appearances, raising concerns about her health. Quoting unidentified friends, The Atlanta Journal-Constitution reported Wednesday that she was diagnosed with a heart malady this spring and has had several small strokes since then before the more serious one Tuesday.
Lowery, former president of the Southern Christian Leadership Conference, which Martin Luther King Jr. helped found in 1957, said he was initially told by someone in King's office that she did not have a stroke, but he was immediately skeptical because the family is "very protective of her condition."
Poet Maya Angelou, a personal friend, also said Wednesday that King had suffered a stroke. She said they spoke two weeks ago by phone and discussed her health problems. Angelou said she planned to go to Atlanta to see her.
"She's my sister friend and I pray for her and her children," Angelou said in New York. "Everybody, please give a good thought, a positive thought for Coretta."
At a June 30 ceremony at the Georgia State Capitol paying tribute to the King family, Martin Luther King III said his mother was "doing well" and was following doctor's orders to limit her activities. He gave no details.
The Alabama-born Coretta Scott was studying at the New England Conservatory of Music when a friend introduced her to King, a young Baptist minister working toward a Ph.D. at Boston University. They married in 1953 and had four children.
After his assassination in Memphis, Tenn., in 1968, she founded the Martin Luther King Jr. Center for Nonviolent Social Change in Atlanta and traveled widely to help foster her husband's dreams.