There were no casualties in the incident, and the TV channel, France's Canal +, resumed broadcasts after a break of several minutes.
The suspected assailant fired twice in the air as he approached the temporary studio, set up on the Croisette seafront for special coverage of the film festival, they said.
"He said to a woman next to him 'if I were you, I wouldn't stick around here'," said Michel Denisot, the programme's presenter.
There was a brief moment of panic in the crowd around the seafront studio, but no-one was hurt.
Marcel Authier, in charge of public security in the local Alpes-Maritimes department, told AFP that the arrested individual was a 42-year-old man.
"It's someone mentally unstable who was immediately arrested," Authier said.
