(Adds detail on investigation, context)
PARIS, Oct 5 (Reuters) - Police raided the headquarters of French media group Vivendi on Thursday as part of an investigation into alleged market abuse involving its purchase of a stake in Italy's Mediaset, a Vivendi spokesman said.
The offices of French bank Natixis in Paris were also being searched, an Italian source with direct knowledge of the matter said. A Natixis spokeswoman declined to comment.
Vivendi said Thursday's raid was the result of an "unfounded and unjust complaint" by former Italian prime minister Silvio Berlusconi, whose Fininvest lodged a criminal complaint last December alleging market manipulation against Vivendi and another with the Italian market regulator Consob.
Berlusconi and his family are the biggest shareholders in Mediaset through Fininvest, with a 41 percent stake in the TV company. Vivendi owns close to 26 percent of Mediaset.
"I can confirm a raid in connection with the complaint lodged by Fininvest for market manipulation," the Vivendi spokesman said.
"I do not know if the police operation is over or not."
The police raids follow a formal request sent by Milan prosecutors to French authorities several months ago.
Vivendi, led by tycoon Vincent Bollore, became Mediaset's second largest shareholder in late 2016 when it bought a 20 percent stake in the Italian broadcaster.
Last December, when Vivendi acquired an 800 million euro stake in Mediaset, two sources said Natixis was one of two banks mandated by the French group to carry out the purchase. (Reporting by Mathieu Rosemain and Maya Nikolaeva in Paris and Manuela D'Alesssandro in Milan; Writing by Richard Lough; Editing by Sudip Kar-Gupta and Alexander Smith)