WARSAW — The Trump administration had delicately said the meeting of five dozen nations in Warsaw would focus on “Middle East security.” But the unmistakable voice of Rudolph Giuliani, an avowed proponent of regime change in Iran, set a different tone.

Hours before the meeting began, Mr. Giuliani, the former mayor of New York and President Trump’s lawyer, told an anti-Iran rally outside Warsaw’s main stadium that Iranian leaders are “assassins, they are murderers and they should be out of power.”

Mr. Giuliani said he was representing an Iranian opposition group, the Mujahedeen-e-Khalq, or M.E.K., not Mr. Trump. But his message belied what American officials had told nervous European counterparts: that the two-day meeting would focus on a range of Middle East security matters.

As Mr. Giuliani spoke, Secretary of State Mike Pompeo was in northeast Poland witnessing live-fire exercises intended to send a message to Russia. And Vice President Mike Pence, joined by the president’s son-in-law, Jared Kushner, was arriving in Warsaw for what amounted to the largest diplomatic gathering that the Trump administration has organized.