Mr. Zarrab testified that it was Zafer Caglayan, Turkey’s economy minister at the time, who said Mr. Erdogan had directed the two banks, Ziraat Bank and VakifBank, to participate in the scheme. Mr. Caglayan “told me that Mr. Prime Minister had given approval for this work,” Mr. Zarrab said through an interpreter, referring to the Iranian trade.

On Wednesday, Mr. Zarrab testified that he had paid tens of millions of dollars in bribes to Mr. Caglayan; he said Mr. Caglayan asked for 50 percent of the profits generated by the scheme, which prosecutors say was being run through a Turkish state bank, Halkbank. On Thursday, Mr. Zarrab said he had also paid bribes to the general manager of Halkbank, Suleyman Aslan, whom he described as being critical to the arrangement.

Mr. Caglayan and Mr. Aslan, who have also been charged in the case, remain at large, prosecutors have said.

Mr. Zarrab testified that he had never paid a bribe to Mr. Atilla — the defendant being tried, who was the deputy general manger of Halkbank.

In Turkey, reaction to Mr. Zarrab’s allegations was muted. Mainstream television and news outlets have largely kept to the government line that the trial is a plot against Turkey, and have not reported the potentially most explosive allegation: that Mr. Erdogan was involved.

Many Turks have turned instead to social media, which has been buzzing about the trial. But some fell quiet after Mr. Zarrab seemed to implicate the president. One Turkish user said on Twitter that he did not dare write what he was hearing — and then deleted even that tweet.