* Cannes already on unprecedented security alert
* Filmmakers to mourn victims of British bombing
* Movie star Farrell says it was "attack against humanity"
By Robin Pomeroy
CANNES, France, May 23 (Reuters) - Filmmakers and fans at Cannes will hold a minute's silence on Tuesday for the victims of the Manchester bombing, condemned by the film festival's organisers as an "attack on culture, youth and joyfulness".
Cannes was already under massive surveillance due to a deadly truck attack on revellers in Nice, just up the French Riviera coast, last July, and gun and bomb attacks on a music concert and football match in Paris in 2015, and its police chief said security measures would be followed "to the letter".
Festival organisers expressed "horror, anger and immense sadness" at the suicide bombing that killed at least 22 people, including children, at a concert by U.S. singer Ariana Grande in Manchester, northern England, on Monday night.
Festival-goers were asked to hold a minute's silence at 3 p.m. (1300 GMT).
Local police chief Yves Daros told Reuters: "We are going to follow the procedures to the letter, without any deviation, to ensure this is going to be a safe festival."
But the bombing was already having an impact at the festival, one of the biggest global entertainment events.
An appearance by racing driver Lewis Hamilton to promote the Disney franchise "Cars" on Tuesday was cancelled "in the wake of last night's tragic attack, and out of respect for the casualties and all of those impacted," according to the publicist.
Colin Farrell, who stars in two of the biggest films in competition at Cannes, said the assault on a concert attended primarily by children and teenagers was "an attack against humanity and it's an attack against society and it's an attack against the hearts and lives and minds of innocent people."
Farrell told Reuters the bombing of what was "a celebration of music and a celebration of coming together in that time-honoured historical tradition of enjoying a creative outlet together as a community - it's just gross." (Addtional reporting by Helena Williams; editing by Mark Heinrich)