A documentary on the Irish rock band U2's dizzying rise to superstardom rocked Canada's largest metropolis as it opened the Toronto film festival.

Fans cheered as Bono and U2 guitarist The Edge appeared for the gala opener of "From The Sky Down," directed by Davis Guggenheim.

It is the first time ever that a documentary kicks off the festival, which runs until September 18. The festival showcases 268 feature films and 68 shorts, including 123 world premieres.

The film, as well as documentaries about Neil Young and Pearl Jam also featured at the festival, "allowed the bands to be reflective on difficult periods in their careers in a meaningful way," lead festival documentary programmer Thom Powers told AFP.

"With hindsight, looking back 20 years people are more candid and willing to talk about the troubled times and the good times. And that's certainly the case in 'From The Sky Down,'" he said.

In the movie, band mates are "talking about a period when they nearly broke up," he explained. "It was very difficult and when you're going through it you don't have perspective."

Guggenheim won an Oscar in 2007 for "An Inconvenient Truth," a documentary on climate change featuring former US vice president Al Gore.

The Toronto event is the biggest film festival in North America, and has traditionally been a key event for Oscar-conscious studios and distributors.

Unlike the Cannes and Berlin festivals, Toronto does not award jury prizes.

But audience favorites -- such as last year's "The King's Speech," which chronicled the reluctant rise to power of a stammering British royal (Colin Firth) after his brother abdicated in 1936 -- often go on to win Oscar nods.

This year, stars including George Clooney, Brad Pitt, Clive Owen, Freida Pinto, Glenn Close, Catherine Deneuve, Rachel Weisz, Salma Hayek and Viggo Mortensen are expected to grace the red carpet.

The festival will also see new work by legendary directors William Friedkin ("The French Connection" and "The Exorcist") and Francis Ford Coppola ("The Godfather" and "Apocalypse Now"), and "W.E.," a feature film directed by Madonna.

Audiences will also be treated to performances by Woody Harrelson as a dirty cop in "Rampart," Juliette Binoche as a journalist researching student prostitution in "Elles," and Michael Fassbender as famed psychiatrist Carl Yung in David Cronenberg's "A Dangerous Method."

Other documentaries include Nick Broomfield's "Sarah Palin: You Betcha."

Powers pointed to last year's festival smash hit "The Promise" about Bruce Springstein for rekindling interest in the rockumentary genre.

"It was such a hit at the festival and made a big impression in the music world; it proved a veteran rocker could still attract new audiences, and the music industry took note," he said. (The Springstein CD boxset with the documentary on DVD was a runaway success.)

For the U2 documentary, director Davis Guggenheim, who worked with The Edge in his 2008 documentary "It Might Get Loud," got his hand on outtakes from a previous U2 documentary "Rattle and Hum."

Guggenheim revisits the band during their tumultuous studio production of "Achtung Baby" (1991) which marked U2's reinvention following its best-seller "Joshua Tree" (1987) and less successful follow-up "Rattle and Hum" (1988).

Also in the music category, "The Love We Make" looks at ex-Beatle Paul McCartney on a New York runway waiting for a flight to London on September 11, 2001 when hijacked airplanes struck the New York twin towers.

Hundreds of filmmakers, actors and journalists were stranded in Toronto during the festival in 2001.

On Sunday, a four-minute short film looking back at the September 11, 2001 attacks in the United States will be shown at the start of every screening to mark the 10th anniversary of the terrorist strikes.