British star Benedict Cumberbatch stars as Assange in the drama, directed by Bill Condon and co-starring Stanley Tucci and Laura Linney.
The closing night gala will be Dan Schechter's Life of Crime, based on Elmore Leonard's story The Switch.
Artistic director Cameron Bailey and TIFF chief executive Piers Handling unveiled the opening night title as well as dozens of films slated for the festival's gala and special presentations programs, about a quarter of the films slated for the overall lineup.
Other hotly anticipated films announced for the lineup include:- John Wells’s adaptation of the acclaimed Tracy Letts’s play August: Osage County, starring Meryl Streep, Julia Roberts, Ewan McGregor, Chris Cooper, Juliette Lewis and Benedict Cumberbatch .
- Justin Chadwick’s Mandela: Long Walk to Freedom, starring Idris Elba.
- The world premiere of British artist-filmmaker Steve McQueen's Twelve Years a Slave, adapted from Solomon Northup's historic book and starring Chiwetel Ejiofor, regular collaborator Michael Fassbender, Benedict Cumberbatch, Paul Giamatti and Brad Pitt.
- Alfonso Curaron’s thriller Gravity, starring Clooney and Sandra Bullock.
- Controversial Cannes Palm d'Or winner Blue is the Warmest Colour.
Some of Canada's top filmmakers will be unveiling their latest work at this year's event, including Don McKellar (The Grand Seduction), Denis Villeneuve (Prisoners), Jean-Marc Vallée (Dallas Buyers Club), Atom Egoyan (Devil's Knot) and Paul Haggis (Third Person).
The initial batch of films revealed also includes one of the final films of actor James Gandolfini (Enough Said). First-time filmmakers will include actor Keanu Reeves, who will screen his directorial debut Man of Tai Chi.
Today's announcement is the festival's first of the season. More lineup revelations are expected in August.
The 2013 Toronto International Film Festival runs Sept. 5-15.