World Book Day goes online for the first time in its six-year history as readers worldwide are given the chance to talk to a range of authors.
Nigella Lawson opened the day's events
Thursday's festival - which focuses on the UK and Ireland - introduces online events to bring readers and writers together.
The international arm of World Book Day will be celebrated on 23 April.
A day-long programme of events includes a live web cast with best-selling fantasy writer Terry Pratchett.
Comedian and author Meera Syal, who wrote the novel and screenplay for the movie Anita and Me, is among the authors involved.
Novelist Malorie Blackman and Michael Rosen - one of the UK's leading children's poets - are also taking part.
Highlights include political commentator and author John Sergeant talking to readers online, SAS expert Andy McNab speaking about his real-life experiences and blockbuster writer Sarah Harrison talking about her favourite books.
Through the web chats, readers will be able to share their own experiences, thoughts and opinions with their favourite writers and fellow readers.
British cookery writer Nigella Lawson launched the festival from the British Library at 1000 GMT on Thursday.
Bringing the festival online coincided with the fruition of government initiative The People's Network, which aims to connect all of the UK's public libraries to the internet.
It means there are 30,000 terminals in 4,000 public libraries across the country, with trained librarians to help people navigate the web.
Authors taking part in live web chats
Book Aid International - the charity that raises cash to buy books for children around the world - is also playing a part in the day's events.
In a new initiative, the charity has organised the twinning of 10 public libraries in the UK with 10 in developing countries.
Children are being invited to the libraries to read the children's book A Life Like Mine, 500 copies of which have been donated by publisher Dorling Kindersley.
"The book focuses on the lives of children in dozens of countries around the world," said spokesman Richard Leggatt.
The date of the international arm of World Book Day - 23 April - has been picked partly because it marks the birth or death of such writers as Miguel de Cervantes, William Shakespeare and the Inca Garcilaso de la Vega.
Australia, Canada, Cuba, Finland, Germany, Greece and Italy are among the countries involved.
In Spain, a comic book called Copy's Adventures will be published to raise awareness about copyright among students and young people while in Sweden 35,000 books are being donated to schools.
In Mozambique 15 paintings by local artists will be sold at a charity auction to raise funds for youth rehabilitation projects through books and sports.
Participants will also be invited to donate new and used books to a stock of five mini-libraries.