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Representative Paul Ryan, the Republican House speaker who is the dreamed-of presidential candidate of many donors and Republican operatives hoping to stop Donald J. Trump, was a featured guest at a Florida dinner of party donors and strategists, an aide confirmed.

Mr. Ryan’s attendance Thursday at the Palm Beach conference, hosted by the hedge fund executive Paul Singer, came as he has repeatedly said he is not running for president. His presence was first reported by Politico.

The gathering had been planned for many months, with the schedule set long in advance. But it meant that Mr. Ryan was present for an event studded by people involved in efforts over the last several weeks to stop Mr. Trump from winning the nomination.

Mr. Singer and other major donors have financed Our Principles PAC, which is advised by the strategist Brian Baker and was founded in late January with the goal of halting Mr. Trump’s rise.

Asked about Mr. Ryan’s attendance, Kevin Seifert, the executive director of Mr. Ryan’s political arm, Team Ryan, confirmed that he was there and said, “His remarks will be in line with what he said this week regarding the presidential nominating contest — that he is not running and is neutral in the process.”

“He will focus on the House G.O.P. agenda that is being developed by the task forces. His participation will be focused on the House, not on the presidential contest,” Mr. Seifert added.

Mr. Ryan has focused on an issues agenda for the Republican Party for many years, and he is now emerging as one of the most visible counterweights to Mr. Trump. Earlier on Thursday, Mr. Ryan criticized Mr. Trump’s statement that there would be riots if the G.O.P. tries to choose someone else at the party convention in Cleveland in July.

“Nobody should say such things in my opinion, because to even address or hint to violence is unacceptable,” Mr. Ryan said

