PARIS--French media group Vivendi SA said Thursday that it is selling a 5.8% stake in U.S. videogame company Activision Blizzard worth about $866 million at current market prices.

The move will cut Vivendi's stake in the company by half and it plans to sell the rest later. The current sale will be carried out through a bookbuilding process and the results released before the opening of New York's Nasdaq market.

Over the past two years, Vivendi has put a number of assets up for sale, such as its French mobile telephone unit SFR and African phone operator Maroc Telecom, in an effort to pay down debt and refocus on media.

Write to Inti Landauro at inti.landauro@wsj.com

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