PARIS — Nokia said on Thursday that its sales fell sharply in the first quarter because of lagging demand in emerging markets for its simplest models, adding to investor anxiety over the company’s ability to execute a turnaround.

Although sales of Nokia’s Lumia Windows smartphones increased by 27 percent, investors focused on the weakness in sales of its basic cellphones, which are the bulk of Nokia’s business.

Nokia, the former cellphone market leader that has tied its future to a smartphone collaboration with Microsoft, reported a 20 percent decline in quarterly sales, to 5.9 billion euros, or $7.7 billion. Shares in Nokia, which is based in Espoo, Finland, fell as much as 6 percent in Helsinki.

Stephen A. Elop, Nokia’s chief executive, said concerns about the progress of the company’s turnaround were overstated. He noted that Nokia’s financial results, and its smartphone business, showed continued signs of improvement.