Some 625 libraries, bookshops, publishers and educational and cultural institutions are taking part in this year's event with over 1,130 programmes under the common theme "The Future of the Book".
The aim of this year's theme is to encourage thinking about the future of the book in the digital age and its links with science, inventions, innovations, new formats and new technologies.
As many as 188 primary schools are also involved, and the programmes include book launches, public readings, workshops, and meetings with writers.
The opening ceremony was held at the National and University Library in Zagreb, with Culture Minister Nina Obuljen and Science and Education Minister Blazenka Divjak attending.
The event formally kicked off with the "launch" of a rocket made of books. A 3D animation showing an astronaut will be projected on the fountains in front of the National and University Library from 8pm to 11pm.
The head of the Croatian Association of Publishers and Booksellers, Miso Nejasmic, said that this was a modest way of showing "where we would like to see our society in the future."