** Shares of ad firms Omnicom OMC.N and Interpublic Group IPG.N fall 2.6% and 0.9%, respectively
** London rival WPP WPP.L reports worse-than-expected decline in organic revenue hurt by weak client spending
** WPP CEO Mark Read said it was a "tough market out there," amid pressure in British market, the U.S. and China
** OMC and IPG are set to complete a $13.25 billion all-share merger this year that would make them the world's biggest ad group
** IPG down 14.3% in the past 12 months, OMC has lost 6% in the same period
(Reporting by Medha Singh)
((medha.singh@thomsonreuters.com))