Analysts said that WPP, currently the world's largest advertising group, and France's smaller firm Havas could attract clients from the two merging firms as they focus on integrating the deal. Some have also suggested the merger could raise conflicts of interests in the enlarged group.

WPP opened up 4 percent while Havas was up 6 percent. Publicis is due to start trading later in the day.

(Reporting by Kate Holton; editing by Neil Maidment)

Stocks treated in this article : HAVAS, PUBLICIS GROUPE, Omnicom Group Inc., WPP PLC