Lennon was killed in 1980, outside his home in New York City.
Many fans visited the nearby memorial to Lennon, named Strawberry Fields after the Beatles song.
The memorial to the slain ex-Beatle and peace activist includes a mosaic donated by the city of Naples, Italy. Also, a plaque lists 121 countries that endorse Strawberry Fields as a Garden of Peace.
In Los Angeles, a public celebration of Lennon's life was held outside Capitol Records in Hollywood.
The birthday celebration got started on Friday in England, where Google UK released a video "doodle" to a Lennon soundtrack.
The Associated Press contributed to this story.