Zohran Mamdani, the new mayor of New York City, called President Trump on Saturday to personally object to strikes the United States had conducted in Venezuela and to the capture of its leader, Nicolás Maduro.

“I called the president and spoke with him directly to register my opposition to this act,” Mr. Mamdani said at an unrelated news conference on Saturday, adding that he told Mr. Trump that he was “opposed to a pursuit of regime change, to the violation of federal and international law.”

The mayor declined to characterize the president’s response when pressed by reporters. His aides said that Mr. Mamdani had initiated the call earlier on Saturday afternoon and that it was “brief.”

“I registered my opposition, I made it clear and we left it at that,” Mr. Mamdani said.

The White House did not immediately respond to a request for comment on the call.

The remarks — and an earlier written statement — were the first time the mayor, a left-leaning Democrat, had clashed with Mr. Trump since taking office on Thursday. But it raised the possibility of more conflict in the days ahead after Mr. Maduro arrives in New York to face federal drug and weapons charges in Manhattan.

Mr. Mamdani has profound differences with the president, and has called him a “fascist.” But after the men had an unexpectedly cordial meeting at the White House in November, Mr. Mamdani had been careful not to directly stoke Mr. Trump’s ire — including in his inaugural address this week.

Mr. Mamdani sought on Saturday to play down how the call might affect that relationship.

“The president and I have always been honest and direct with each other about places of disagreement,” he said. “New Yorkers have elected me to be honest and direct and always to do so with the understanding that my job is to deliver for the people who call this city home.”

Mr. Mamdani will likely have little influence over what happens to Mr. Maduro in federal custody, even if the leader and a potential trial are being held within the city limits of New York. The mayor seemingly acknowledged these limits when speaking with reporters.

“It is my responsibility that whatever actions the federal government takes, that they have a minimal impact on the day-to-day lives of New Yorkers,” he said.

Mr. Mamdani has been an outspoken critic of American support for Israel’s military campaign in Gaza, but before his comments on Saturday, he had not had much to say about Venezuela or U.S. policy in Latin America. Both Mr. Mamdani and Mr. Maduro identify as socialists, though they have no relationship.

In labeling the American action unlawful, Mr. Mamdani echoed numerous other Democrats. But where other leaders in his party called Mr. Maduro an “illegitimate dictator,” Mr. Mamdani did not comment on his record or standing in Venezuela.

“This blatant pursuit of regime change doesn’t just affect those abroad, it directly impacts New Yorkers, including tens of thousands of Venezuelans who call this city home,” the mayor said in his written statement.

“My focus is their safety and the safety of every New Yorker,” he added, “and my administration will continue to monitor the situation and issue relevant guidance.”