Mourners sang and laid flowers on Tuesday on Central Park's candlelit memorial to John Lennon on the 40th anniversary of his murder in New York City.
Lennon's widow, Yoko Ono, marked the moment with a call for gun control.
"The death of a loved one is a hollowing experience," tweeted the artist, 87, who still lives in the Manhattan building where her husband was murdered.
"After 40 years, Sean, Julian and I still miss him," she said, before quoting the 1971 song she co-wrote with Lennon that became the best-selling single of his solo career.
"Imagine all the people living life in peace."
Ono, who was with her husband when he was shot, tweeted an image of the former Beatle's shattered and bloodied spectacles with the words: "Over 1,436,000 people have been killed by guns in the US since John Lennon was shot and killed on December 8, 1980."
She included hashtags such as #guncontrol now and #endgunviolence, and one for the National Rifle Association, the powerful US organisation that has for decades campaigned to eliminate firearms control legislation.
At 40, Lennon returned to songwriting shortly before his death, having taken a five-year hiatus to raise his young son Sean, who on Tuesday posted a candid family photo on social media.
He and Ono were returning home to New York's famous Dakota Building, across from Central Park, when disgruntled Beatles fan Mark David Chapman shot Lennon dead.
He was rushed to the emergency room in the backseat of a police car, a harrowing experience detailed in The New York Daily News the next day.
Officers stood in disbelief in the emergency room, it said, as: "John Lennon, whose music they knew, whose music was known everywhere on Earth, became another person who died after being shot with a gun on the streets of New York."
After Lennon's murder, Ono committed to preserving his memory, paying for the Strawberry Fields memorial in New York’s Central Park, which became a site for fans and mourners across the globe.
By midday on Tuesday, a shrine for Lennon featuring roses and daisies, photos and a small Christmas tree cropped up on the "Imagine" mosaic that anchors the memorial.
Fans sang and danced as a guitarist strummed tunes including The Beatles' Norwegian Wood (This Bird Has Flown).
Tepper Saffren, who goes by "Sergeant Tepper" and regularly visits the corner of the park to sing Beatles covers, said the power of Lennon's lyrics still rang true today.
"When you play any Beatles song, everybody's got something that they can tap into," Tepper, 28, told AFP.
"And usually it's a good feeling, or at least a beautiful feeling."
Lennon's former songwriting partner Paul McCartney posted: "A sad, sad day but remembering my friend John with the great joy he brought to the world."
Beatles drummer Ringo Starr called on the world's radio stations to play Strawberry Fields Forever at some point on Tuesday.
Some faces in the crowd on Tuesday showed that Lennon's influence was still touching the youths of today.
New Jersey resident Gabriela Parra, 20, said she aimed to visit each year "to celebrate his life and legacy".
"A lot of people think that the music of The Beatles will at some point dissipate because it's so old, but young people still connect to the message and the music," Parra said.
"You have celebrities that come and go but there's no real icons, nobody that wants to be the image of our generation.
"Nobody will ever compare to John Lennon."