During Mr. Trump’s first term, Mr. Musk had said there should be “no tariffs at all either way” between the United States and Britain.

Mr. Musk’s appearance at the League’s meeting, in which he warned of the dangers of censorship and mass immigration, came as he continued to use his influence to bolster far-right forces across Europe.

On Friday, Mr. Musk and Mr. Trump had expressed support for Marine Le Pen, the French far-right leader, who was convicted this past week on embezzlement charges and disqualified from running for public office. Earlier this year, Mr. Musk publicized the German far-right Alternative for Germany party and weighed into Italy’s immigration debate, prompting the country’s president to rebuke him.

On Saturday, he appeared at the party conference with Matteo Salvini, the leader of the League party. During his brief speech, Mr. Musk also promoted the activities of his cost-cutting Department of Government Efficiency, supported Mr. Trump’s position on the war in Ukraine and attacked what he called the overregulation of the European Union.

Maggie Haberman contributed reporting.