Simon Cowell, Amanda Holden, David Walliams and Alesha Dixon posed for photographs and greeted fans outside the city's Waterfront Hall as the show visited Northern Ireland for the first time in its eight-year history.
Ahead of auditions, Cowell tweeted: "Hello. I am in Belfast. First day of britains got talent. Have just seen a cat backstage trying to play the guitar!"
David Walliams replied to Cowell later: "Can you get a bloody move on? It's three o'clock in the afternoon!"
Dixon - who gave birth to her daughter Azura Sienna in October - spoke about returning to Britain's Got Talent earlier this month.
She explained: "It's full on but I'm looking forward to it. When we all get together it is like going back to school. Everyone turns into little kids again. I can't wait to see what it brings this time."
According to The Sun, the new series of the talent contest will see the introduction of a "Golden Buzzer", which will allow each judge to send through an act to the live shows whatever everyone else on the panel thinks.
Britain's Got Talent returns to ITV later this year.
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