Plains, Georgia - Jimmy Carter celebrated his 100th birthday on Tuesday – the first-ever US president to reach the century mark and another extraordinary milestone for the one-time peanut farmer who found his way to the White House.

Carter's longevity – he began hospice care at his home in Plains, Georgia, more than 19 months ago – has defied all expectations.

No longer regularly appearing in public, the Democratic ex-president is spending his birthday in the home he and his late wife Rosalynn had built in Plains in the 1960s. Celebrations include a lunch with some 20 members of his extended family, according to the Atlanta Journal-Constitution.

President Joe Biden, in a video posted on his official X account, hailed Carter as a "beloved friend" and "one of the most influential statesmen" in US history.

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"Your unwavering belief in the power of human goodness continues to be a guiding light for all of us," he said.

The White House lawn was decorated Tuesday with a sign that read "Happy Birthday President Carter."

Britain's King Charles III sent a private message of congratulations expressing "great admiration for the president's lifetime of duty and public service," a Buckingham Palace spokesperson said.

And Georgia Governor Brian Kemp, currently overseeing the response to a major storm disaster, proclaimed "Jimmy Carter Day" across the state.

Beyond serving as commander in chief for a single term from 1977 to 1981, Carter worked as a global mediator, rights activist and elder statesman, founding the well-respected Carter Center in 1982 to pursue his vision of world diplomacy.

His presidency included the 1978 Camp David Accords between Israel and Egypt, the establishment of diplomatic relations with China following a rapprochement initiated by President Richard Nixon, and the return of control of the Panama Canal to Panama.

But his administration hit numerous snags, including the Iran hostage crisis and a renewed oil crisis in 1979-1980, which saw Ronald Reagan sweep into power in a landslide.

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"We are thrilled that the president from this little town is going to be the only president to ever live to be 100 years old," Jill Stuckey, superintendent of the National Park Service's Jimmy Carter historical site and longtime family friend, told AFP.

She spoke from Plains High School, where Carter graduated in 1941 and which now serves as the visitor center for the national park that features sites from his early life, including the nearby peanut farm where he grew up.

On Tuesday, the school held a naturalization ceremony for 100 new US citizens, who placed hands over their hearts and recited the pledge of allegiance.

"He did so much for America," said Adriana Vickers (46) originally from Brazil.

"He's for all nations," Carlene Ford, another newly naturalized citizen, from Jamaica, told AFP. "He's for everybody, he's for the people."

A flyover of four F-18 jets and two vintage planes graced the skies over Plains as part of the town's celebrations.

A devout Baptist and self-described "born-again" Christian, Carter is remembered for a moralistic streak, but also his civility, admired even by detractors across the aisle.

According to family, Carter remains keenly interested in politics and was highly motivated to make it to 100 and to vote in the November election for fellow Democrat Kamala Harris.

"He will be voting by mail-in ballot," Stuckey told AFP, adding, "he's always been very politically active, and nothing has changed with that respect."

At the naturalization ceremony, attended by the mayor, those in attendance were encouraged to register to vote.

In Plains, like in many rural places with strong evangelical Christian leanings, far more yard signs support Republican Donald Trump than Harris.

But it is also not uncommon to see a sign celebrating Carter's centennial birthday next to one supporting Trump.

Carl Lowell, who has lived in Plains since infancy, said he tries not to get involved in politics because it is "so divisive" right now.

Like most residents of Plains, the 59-year-old retired firefighter is linked to Carter in various ways – he says his grandfather helped build the Carters' house and that he himself even went dove hunting with him once.

"Jimmy's a good man, he's a godly man, and that's what people like about him," he said.