"His music speaks to people of any nation, any age, and that's why I think so many young people now who never would have known him still find him so appealing," said Karen Kriendler Nelson, 69, who lives nearby and often visits the mosaic that spells out Lennon's song "Imagine."
She and her Maltese dog, Pino, joined a group of fans who sang the lines, "Imagine there's no countries/ It isn't hard to do/ Nothing to kill or die for/ And no religion too/ Imagine all the people/ Living life in peace ..."
Joan Acarin and his wife, Laia, visited the memorial from Spain.
"The values Lennon defended are still alive," said Joan Acarin, a 41-year-old attorney from Barcelona. "It's the idea that we do not have to fight wars."
Fans began arriving on Friday, spilling onto the sidewalk of Central Park West, where Lennon and wife Yoko Ono lived in the famed Dakota building for nine years. He was shot to death by a deranged gunman as he came home on the evening of Dec. 8, 1980.
Police erected barricades to contain the crowd alongside passing traffic.
This year, the memorial to the slain ex-Beatle and peace activist includes a mosaic donated by the city of Naples, Italy. A plaque lists 121 countries that endorse Strawberry Fields as a Garden of Peace.
The 2.5-acre site was created by Ono and named after the Lennon song, which also observes that "living is easy with eyes closed, misunderstanding all you see."
The birthday celebration got started early Friday in his native England, where Google UK released a 32-second video "doodle" with an "Imagine" soundtrack. The interactive electronic art generates a butterfly and a flower - reflecting Lennon's devotion to world peace.
In Liverpool, Lennon's first wife, Cynthia and, their son, Julian, unveiled a sculpture to celebrate his life.
Hundreds of people gathered at the city center's Chavasse Park to watch the pair cut a ribbon to reveal the statue, called "Peace and Harmony." The sculpture, which features a colorful globe with doves flying above it, was designed by 19-year-old American artist Lauren Voiers.
The two held hands and joined the crowd in singing John Lennon's "Give Peace a Chance."
"I think the mourning is over for John. I think it's time to celebrate," said Cynthia, 71. "Think about his life that was positive and good and just enjoy that."
She was married to John Lennon from 1962 to 1968.
In New York, planned celebrations include a Saturday evening benefit concert at the Society For Ethical Culture, a short walk from Strawberry Fields. The proceeds will go to the human rights organization Amnesty International.
Capping the New York remembrances would be a Central Park screening of a documentary detailing Lennon's life in the city. Titled "LENNONYC," the new public television film to be shown at 7 p.m. in the park's Rumsey Playfield, with picnic-style seating on the ground.
Ono marked her late husband's milestone birthday in Iceland with a lighting of the Imagine Peace Tower, which shines a beam of blue light into the sky, followed by a concert by the Plastic Ono Band. Ono dedicated the tower to Lennon in 2007.
She also presented awards to people who have contributed to peace, included Alice Walker, the author of "The Color Purple."
"Like millions around the world, I deeply loved John," Walker said as she received the award in Reykjavik.
The other winners were filmmaker Josh Fox, who made the documentary "Gasland"; author and activist Michael Pollan and food safety advocate Barbara Kowalcyk.
Just before Lennon was killed, the couple collaborated on a last album "that was so different from anything he did before," said David Edwards, a college student in Kentucky who drove 14 hours to New York City to pay tribute.
The 22-year-old found a different way to honor the slain Beatle in the bustling crowd of admirers: He sat alone on a bench with earphones on, listening to Lennon's music on his iPod while reading his book "Skywriting By Word of Mouth."
"What gets me is his humanity," Edwards said. "He was one of the first superstars who showed that he was vulnerable - he was Everyman."