Former US president Jimmy Carter, 99, made a rare public appearance Tuesday to join his successors Joe Biden and Bill Clinton and all five living first ladies at the memorial service for his beloved wife Rosalynn.

A frail-looking Carter briefly left hospice care to attend the service at a church in Atlanta, arriving in a wheelchair with a blanket on his lap that was embroidered with a picture of his wife's face.

Carter did not speak, but the couple's children and grandchildren celebrated the "life well lived" of Rosalynn Carter, a humanitarian and mental health advocate who redefined the role of the modern First Lady.

"My mother was the glue that held our family together," said their son James "Chip" Carter, who kissed his father on the head and laid his hand on his mother's flower-covered coffin.

Biden and First Lady Jill Biden had a "special moment with President Carter" before the service, White House Press Secretary Karine Jean-Pierre told reporters on Air Force One.

Rosalynn Carter died on November 19, aged 96, at the couple's home in Plains, Georgia, after a 77-year marriage that became a legend in US politics.

- 'In love' -

Fighting back tears, their daughter Amy Lynn Carter read out a love letter her father wrote to her mother while he was in the US navy three quarters of a century ago.

"My mum spent most of her life in love with my dad," she told the congregation at the Glenn Memorial Church.

Sitting in the front row of the church were Biden -- who had backed Carter before the Democrat's 1976 election win -- and First Lady Jill Biden.

Alongside them were former president Bill Clinton, who was in office from 1993-2001, and former secretary of state Hillary Clinton; and former first ladies Melania Trump, Michelle Obama and Laura Bush.

The service included her favorite music, including a performance of the John Lennon song "Imagine" by husband-and-wife US country music stars Tricia Yearwood and Garth Brooks.

The turnout showed the high regard in which Rosalynn Carter was held in the United States.

Celebrated as an active first lady who championed then little-discussed issues of mental health, Rosalynn Carter's reputation only grew, along with her husband's, once they left the White House.

Losing his 1980 reelection bid to Republican Ronald Reagan, Jimmy Carter was widely dismissed as a failure.

However, the couple went on to build a global network of charity activities and earned plaudits for their humble lifestyle.

- 'Equal partner' -

Chip Carter hailed his mother for helping him overcome his own drug and alcohol addiction, saying the former first lady made a "positive difference in people's lives.

Joe and Jill Biden flew from Washington early Tuesday -- and gave the Clintons and Michelle Obama a lift on the presidential plane Air Force One, the White House said. Melania Trump did not take the same plane, it added.

The more private funeral on Wednesday in Plains will inevitably shift attention to the health of Jimmy Carter, who is the oldest living US ex-president in US history and moved into hospice care 10 months ago.

Presidential funeral services typically are attended by all living presidents -- Carter attended the Washington memorial for George W. Bush in 2018 -- providing a moment for national unity.

With Biden and Donald Trump likely to face each other in a 2024 rematch of their bitter 2020 election battle, however, even the solemn occasion of a Carter presidential funeral would be scrutinized for signs of tension.

The Carters married in 1946 and held the record of longest-wed presidential couple.

When she died, Jimmy Carter said in a statement that she'd been "my equal partner in everything I ever accomplished."

Throughout Jimmy Carter's long political career, his wife was at the heart of his campaigns. And during the 1977 to 1981 White House term, Rosalynn Carter worked to raise the status of the first lady's office, even sitting in on cabinet meetings.

She met Jimmy Carter in 1945 while she was in college and he was on leave from the US Naval Academy in Annapolis.