People lined up for blocks outside the church's historic sanctuary, waiting for hours in freezing rain.
King's children entered around noon, as did TV talk-show host Oprah Winfrey.
Across the street, at least 1,000 people filled the church's newer facility for a musical tribute featuring Gladys Knight.
Mary Howard-Hamilton, a college professor from Bloomington, Ind., drove eight hours to Atlanta and then stood in the rain for five more to be among the first to view King's body at the church.
"It's almost like the torch was passed when I walked past her," Howard-Hamilton, 51, said. "I felt empowered. I'm gonna step up now. This fight's not over."
King, who died Jan. 30 at age 78, was lying in honor at the church where her husband, the Rev. Martin Luther King Jr., shared his dream from the pulpit in the 1960s.
Inside the silent sanctuary, mourners filed slowly past the casket, some lingering a moment before moving on. A shroud of flowers blanketed the lower half of the casket, and wreaths stood on either side, decorated with roses, King's favorite flower.
At least 6,000 adults and children passed through the sanctuary in the first three hours, officials said.
During the weekend, some 42,000 mourners had walked past King's open casket at the state Capitol, where she became the first woman and the first black person to lie in honor there. It was a striking contrast to the official snub her slain husband had been given by then-Gov. Lester Maddox, an outspoken segregationist.
President Bush and former President Clinton lead the list of dignitaries expected to attend her funeral on Tuesday, to be held at New Birth Missionary Baptist Church, a megachurch in Lithonia where the Kings' youngest child, Bernice, is a minister.
Civil rights leaders also planned to memorialize King during a service later Monday that was expected to include the Rev. Jesse Jackson, the Rev. Al Sharpton, Rep. John Lewis, former Atlanta Mayor Andrew Young and the Rev. Joseph Lowery.
Workers were expected to erect a temporary, marble mausoleum Monday for King near that of her late husband at the King Center which she founded in his name, said Brandon Shields, president and owner of Marietta-based Roberts-Shields Memorial Company. It will be used temporarily until a structure identical to the slain civil rights leader's can be built.
King suffered a stroke and heart attack last year and had been diagnosed with cancer, but she appeared to be making steps toward recovery, her children said Sunday.
She was to begin treatment at an alternative medical clinic in Mexico the day she died.
"It came as a tremendous shock to us. We had no idea," eldest daughter Yolanda King said at a news conference. "She was walking with a cane, she was speaking more words ... there was clearly progress happening."
Yolanda King said family members had thoroughly researched the clinic and "were stunned when we found out there were problems and challenges there." Mexican authorities shut down the clinic days after King's death, saying it had carried out unproven treatments and unauthorized surgeries.
"We're missing her like crazy, but we're just so thankful that we had her as long as we did," Yolanda King said. "She's been released and we feel so strongly that she has reconnected with our father."