Lennon followers laid wreaths outside the New York apartment block where the musician was shot dead on December 8, 1980.
His widow, Yoko Ono, emerged from the Dakota building to lay white roses on the "Imagine" mosaic that is the centrepiece of Central Park's Strawberry Fields tribute to the singer.
Hundreds of people watched Ono as a man played Beatles music on an acoustic guitar for the silent crowds.
People continued to line up to see the mosaic late into the night, braving freezing temperatures and singing songs by candlelight.
obs: the Dakota building is the same where Roman Polanski did the movie Rosemary's Baby..His wife was murdered in that time by Charles Manson (i guess that his name is right) ..who wrote in the wall "PIGS" with Roman's wife blood.
In the English city of Liverpool, where Lennon was born 65 years ago, more than 1,000 messages to the ex-Beatle, including emails from across the globe, were released on white balloons from the city's Albert Dock.
"I just wrote 'Merry Christmas John' on my balloon," said nine-year-old English fan James Andrews, one of the youngest fans present.
"I love The Beatles and especially John Lennon. My Mum and Dad let me take the day off school to come here," he said.
A short service was held at Lennon's statue in the city, near the Cavern Club where The Beatles, Lennon's former group, played some of their earliest concerts. A memorial service was also held at a Liverpool church.
At the Cavern bar in Buenos Aires, some 30 Lennon imitators sang Beatle songs to mark the anniversary.
Japan marked the anniversary with special radio broadcasts and a free concert at a museum in the Tokyo suburb of Saitama that was built with Ono's blessing.
In Cuba, a concert is scheduled for Sunday at a Havana park where President
Fidel Castro inaugurated a life-size bronze monument of Lennon five years ago.
But New York, Lennon's adopted home, was the focus of the commemorations.
The US Sirius Satellite Radio network hosted a four-hour "Lennon Live" tribute featuring performances from New York and London by rock and pop stars Dave Matthews, Paul Weller, Dr. John, Daryl Hall, Stereo MC's and Lulu.
A steady stream of fans started going to the Dakota building early in the day.
Fans were to hold a candlelight vigil in Strawberry Fields from 10:50 pm (0350 GMT Friday), the time when Lennon was shot by Mark Chapman.
Chapman had asked for Lennon's autograph hours before. "Mr. Lennon!" he shouted before aiming a handgun and firing five bullets at the rock world's preacher for peace.
Fans were angry that the New York authorities would not extend the normal 1:00 am curfew in the park so the vigil could go on through the night.
Lennon's widow was one of four people honoured at the annual dinner of the New York chapter of the National Academy of Recording Arts and Sciences on the eve of the anniversary.
"This week has been hard. Depressing," said Ono, 72. "Now I'm smiling. I'm here. I see all your faces. John would have been so happy that you acknowledged his achievements. This means so much to us. It would have made him happy that you acknowledged our partnership."
A quarter century after Lennon's death, his family, friends and fans are still struggling for control of his legacy.
The anniversary has been marked by exhibitions, memoirs, album re-issues, documentaries and even a Broadway musical and a looming film.
Ono remains the gatekeeper of the Lennon legacy and chief protector of his posthumous image.
In the run-up to the anniversary, other voices have sought to offer a differing view of Lennon, however.
In her memoir "John," published in October, Lennon's first wife, Cynthia Lennon, wrote of the "pain, torment and humiliation" she suffered in her marriage.
And in a statement for the 25th anniversary, Lennon's son, Julian, admitted to "very mixed feelings" about his father.
"He was the father I loved who let me down in so many ways," he said.
Meanwhile, Chapman is serving a 20 years to life prison sentence. His parole appeals have been turned down three times.
In an interview recorded more than 10 years ago, Chapman described how he could not stop himself shooting Lennon. "It was like a train, a runaway train, there was no stopping it. No matter -- nothing could have stopped me," he said.
