Tate Modern mounted a huge helium model to mark the day
Authors who took part in the "read and ride" event included Children's Laureate Anne Fine, Anthony Horowitz, creator of teenage spy Alex Rider, and Liccy Dahl, widow of Roald Dahl.
The promotion was just one of a range of events to mark World Book Day in the UK, where celebrations were brought forward to fit in with the school term.
The other 30 or so countries expected to take part in World Book Day will do so on the traditional day of 23 April - Shakespeare's birthday.
Children from 29 London boroughs and their parents filled the London Eye, which was set aside for the occasion.
Nine-year-old Sam Reinders joined the celebration. He said: "It was great. I'm going to make a little book about it when I get home."
The day is used to encourage the public to explore the pleasures of reading, and raises money to send books to developing countries.
Last year, more than £40,000 was raised by the day's designated charity Book Aid International, allowing it to send books to children in the world's poorest countries.
Stars
Among the most spectacular events to be staged in the UK was the mounting of a giant helium filled model at London's Tate Modern museum.
The model showed a pair of eyes and six metre long lips reading out loud.
Celebrities such as Anne Robinson, Frank Skinner, Ben Elton, Stephen Fry, and Denise Lewis have also been involved in World Book Day.
They feature in the nationwide Get Caught Reading poster campaign which aims to raise the profile of books.
Shakespeare
Around the UK 13 million children received a gift token to celebrate World Book Day.
Every child in full-time education will be able to exchange the £1 token for one of five specially published books by leading children's authors, or another book of their choice.
Bookshops, libraries, arts centres, schools and colleges have been hosting literary events and visits from authors.
World Book Day was set up in 1995 by UNESCO - the United Educational Scientific and Cultural Organisation.
As well as being Shakespeare's birthday, 23 April is St George's Day.
It was chosen for World Book Day because of a tradition in Catalonia, Spain where people celebrated the saint's day by giving a book and a rose to loved ones.