Swiss jeweller Chopard has loaned luxurious jewellery to celebrities in Cannes for the film festival, including actress Julianne Moore, model Cara Delevingne and singer Lana Del Rey.
It is unknown whether the jewels in the pictures below were among the items stolen by thieves.
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The threat level has been raised to critical by MI5's Counter-Terrorism Analysis Centre.
Prime Minister Theresa May announced the decision from Downing Street.
A man, apparently armed with a small axe and a large stick, was detained close to the event and police cleared the area as a precaution.