Publicis reported solid first quarter sales figures Thursday but cautioned that the global ad market will taper in the upcoming months before showing improvement in the second half of the year.

The Paris-based ad group, the world's third-largest by revenue, said it posted top-line sales of €1.45bn in the three months ending in March, a 13 percent advance on the same period last year. Organic sales growth, which excludes any acquisitions by the group, was around 4.1 percent.

CEO Maurice Levy said he expects a slowdown in sales in the second quarter of this year as clients move ad budgets in the second half to better match the twin demands of the London Olympics and the 2012 US Presidential elections.

"We should see a deceleration of growth in the second quarter and a stronger second half compared with the first," he said.

Mr. Levy said he expects the group's operating margin, a measure of profitability, to be around 16 percent this year, broadly in line with last year's improvements, thanks to significant cost-cutting. He does not, however, anticipate any large-scale M&A activity after two medium-sized deals were completed earlier in the year.

Publicis shares fell sharply in the opening minutes of trading, declining nearly 2 percent to trade at €40.36. The stock is up around 14 percent so far this year.