By Sam Schechner 

PARIS--French conglomerate Groupe Bolloré said Thursday it has again boosted its stake in Vivendi SA, bulking up the influence of chairman Vincent Bolloré as he faces a challenge from an activist shareholder.

Groupe Bolloré, which is controlled by Mr. Bolloré, said that it has increased its stake in Vivendi to 10.2% from 8.2% through the purchase of nearly 30 million shares for a total price of EUR632 million ($689 million).

The slug of new shares tightens Mr. Bolloré's grip on the French firm as he faces a tough challenge from a U.S. hedge fund that wants him to boost Vivendi's cash returns to shareholders after selling billions of euros in assets that accounted for more than half of its revenue.

P. Schoenfeld Asset Management, which owns around 0.8% of Vivendi, came to Paris earlier this week to pitch other shareholders on a plan that includes a EUR9 billion special dividend for shareholders. Vivendi's board agreed this week to put three resolutions on dividends and voting to shareholders, but encouraged shareholders to abstain or to vote against them.

Write to Sam Schechner at sam.schechner@wsj.com

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