After the ceremony, the actress said she was touched by the award but felt uncomfortable at the idea of taking credit for Oscar winner Farhadi's Paris-set family drama.
"It is special to get a best performance prize; it is for me and I cannot imagine getting something just for me.
"I would be nothing if there weren't other actors, the director photography, and all the members of the crew," she told reporters.
"It is as if the film is being reduced just to me and I can't envision that," she added, speaking in French.
The actress -- who was born in Argentina and speaks fluent Spanish as well as English -- is married to Michel Hazanavicius, director of "The Artist" which swept the 2012 Oscars with five awards including Best Picture.
Speaking earlier in the festival, she said she received offers from Hollywood on the back of the success of "The Artist".
But she said the chance to keep working in Paris with "one of the world's best directors" was more tempting.
She said the shifting perceptions in Farhadi's films, where things are never quite what they seem, had drawn her to the character of Marie.
"For an actress it was quite an extraordinary experience -- things appear true and then turn out to be completely different," she said.
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