Late Beatles star and musical legend John Lennon , 9 October, and various events marked the day.
The musician, who was shot dead at the age of 40 in New York in 1980, has continued to have an enduring influence on popular culture long after his murder.
Following a chance encounter with his future bandmate Paul McCartney at a church fete in 1957, he went on to change the course of musical history with a body of work which has stood the test of time.
After helping to propel The Beatles to international stardom and securing their place as one of the world's biggest ever bands, Lennon continued to enjoy a varied career.
Following the band's breakup in 1970, the Liverpudlian released his debut solo effort John Lennon/Plastic Ono Band in the same year, which included the likes of Mother and Working Class Hero.
Less than a year later came Imagine, whose title track has become inextricably linked to his legacy.
The Strawberry Fields memorial to the musician in New York's Central Park, where Lennon's ashes are scattered, features a mosaic bearing the word.
Lennon's landmark birthday is being marked with a pop-up television channel called LENNON80 which will feature special programming.
It will feature Bed Peace, the documentary charting Lennon and Yoko Ono's Bed-In For Peace in Montreal, Canada, in 1969, as well as their 1971 interview on Parkinson.
Universal Music Group launched the channel on Sky, Virgin and Freeview from today until October 15.
His birthday was also marked in Liverpool, where the council has launched a worldwide contest to find another great songwriter.
A collection of previously unseen pictures of Lennon in New York is also going on display at The Beatles Story in Liverpool.
Earlier this week, Elton John said that if Lennon had lived he probably would have won the Nobel Peace Prize.
Speaking to Lennon's son Sean, who is his godson, to mark the milestone in a BBC Radio 2 special, Elton John said: "I think if your dad had still been alive he would have definitely been, maybe won the Nobel Peace Prize or something.
"That was what your dad was, he wanted to bring people together. He was a uniter and he was prepared to go to any lengths to make people see what his point was."
Paul McCartney marked the birthday of his late Beatles band mate by sharing a black and white photograph of the pair together.
Sharing a photograph on Instagram, McCartney wrote: "I love this picture, it reminds me of the bond between us. Happy 80th John. Love Paul".
Beatles drummer Sir Ringo Starr also remembered Lennon.
He posted on Instagram: "Let's celebrate John's 80th birthday with come together Friday, 9 October I still miss you man peace and love to Yoko Sean and Julian."
Yoko Ono, Lennon's wife, posted a short video showing New York's Empire State Building lit up in blue, and with a peace sign, in honour of the day.