Dozens of festival staff led by director Thierry Fremaux stood on the red steps of the Palais des Festivals for a minute's silence.
The attack targeting young fans at a show by US singer Ariana Grande cast a shadow over celebrations for the 70th birthday of the world's biggest film festival, where many of the A-list elite are gathered.
A fireworks display due in the southern French resort Tuesday evening has been cancelled after Monday night's stadium attack, which left 22 dead and has been claimed by the Islamic State group.
Dozens of festival staff led by director Thierry Fremaux stood on the red steps of the Palais des Festivals for a minute's silence, joined by Oscar nominee Isabelle Huppert.
"The Manchester victims were doing nothing that we don't do here -- being together, enjoying the freedom to love and share their passions," Fremaux said.
Beachside restaurants near the festival also turned off their music as diners fell silent.
Festival organisers described the bombing as "yet another attack on culture, youth and joyfulness, on our freedom generosity and tolerance, all things that the festival and those who make it possible -- the artists, professionals and spectators -- hold dear."
Security is tight in Cannes, just 10 months after the jihadist truck attack in nearby Nice that killed 86 people.
Unprecedented measures have been taken to protect the festival, with large concrete barriers put up in a bid to prevent another vehicle assault and snipers positioned on the roof above sensitive sites.
Cannes Mayor David Lisnard offered his "heartfelt thoughts" to the Manchester victims, saying on Twitter: "The threat is at maximum. We will stay on maximum alert to prevent it and protect everyone, residents and festival-goers, from the risk of an attack."
Stars including Nicole Kidman, Will Smith, Colin Farrell and Selma Hayek joined directing giants such as Cannes jury chief Pedro Almodovar and Ken Loach earlier Tuesday for a "family photo" to mark the festival's 70th birthday -- many snapping their own selfies for posterity.
A host of celebrities -- the guest-list is top secret -- were due to attend an anniversary bash in the evening.