The protests against President Aleksandar Vucic of Serbia had been growing in intensity and size when an unusual guest showed up in its capital this month to meet with the embattled European leader: Donald Trump Jr., the oldest son of President Trump.

The quick visit by Mr. Trump, which included a meeting with Mr. Vucic to talk about U.S. foreign aid to Serbia, came as the Trump family and Jared Kushner, the American president’s son-in-law, were moving ahead with plans to build a Trump International Hotel in Belgrade, the first such property in Europe.

The hotel is slated to be built atop the site of the former Yugoslavian Ministry of Defense headquarters, which was bombed by NATO 26 years ago on land now owned by the Serbian government. Opposition leaders in Serbia have criticized the agreement and called for it to be terminated, raising the prospect that the deal could be scuttled in a change of power.

Mr. Trump used the visit as an opportunity to express his support for Mr. Vucic — a trip that offered perhaps the most explicit mixing so far in President Trump’s second term of U.S. foreign policy and the Trump family’s financial interests.

On Wednesday, the Serbian prime minister resigned, effectively bringing down the ruling party and forcing Mr. Vucic to form a new government or hold new parliamentary elections later this year, creating more uncertainty there.

A spokesman for Donald Trump Jr. dismissed any suggestion that his visit created a conflict of interest. The spokesman said the trip had been driven by a plan to interview Mr. Vucic for Mr. Trump’s podcast, not to step into foreign relations issues or the real-estate deal.

“Don hosts one of the biggest political podcasts in the world and was in Serbia strictly in his capacity as a podcast host for an interview,” Andy Surabian, the spokesman, said. “He was in and out of the country in less than eight hours and at no point had any discussions with anyone relating to Trump Org.”

The visit, according to two individuals briefed on the plan, was arranged by Brad Parscale, a former campaign manager for President Trump.

Mr. Parscale, an executive at a conservative podcast and radio broadcasting company, also founded a political campaign consulting firm. He had pitched advising Mr. Vucic during his 2022 re-election campaign, but has asserted he did not get hired.

Mr. Vucic is now facing one of the biggest tests of his nearly eight years as president. Protests against his administration erupted in November after the collapse of a concrete structure atop a railway station walkway that killed 15, an accident that demonstrators blamed in part on government corruption.

The visit by Mr. Trump last week had brought a brief pause in those troubles and immediately became national news in Serbia, with Mr. Vucic and his top advisers pointing to it as a sign that the Trump administration supports Mr. Vucic, despite the growing protests in the streets of the capital.

“A cordial conversation with Donald Trump Jr., the son of U.S. President Donald Trump about bilateral relations between Serbia and the U.S.A. and current topics that shape the global political and economic scene,” Mr. Vucic wrote in a social media posting after the meeting.

Marko Djuric, Serbia’s foreign affairs minister, added in a television interview after Mr. Trump’s visit that the presence of President Trump’s son “provides great momentum for an excellent start to relations with the new administration.”

Others in the country had quite a different view.

“The son of President Trump is here to try to give Vucic a helping hand,” said Dragan Jonic, an opposition-party member of Serbia’s parliament. “It is obviously a conflict of interest, as Vucic is trying to hold on to power and the Trumps want to keep their real estate deal alive.”

Mr. Vucic’s government signed an agreement last May with Affinity Global Development, a company set up by Mr. Kushner. The company plans to invest $500 million to build a 175-room Trump hotel with 1,500 luxury apartments and other amenities at the former defense ministry site in Belgrade.

“We are thrilled to expand our presence into Europe,” Eric Trump, another of President Trump’s sons, said in January, when the inclusion of a Trump International Hotel to the project was first publicly announced. Eric Trump is the lead family member running the real-estate company.

But Donald Trump Jr. is also an executive vice president at Trump Organization, which operates the family’s hotels, golf courses and other assets, and is helping with planning for the Serbian hotel project.

Two individuals who had been briefed on Donald Trump Jr.’s travel, but who spoke on condition of anonymity because they were not authorized to discuss it publicly, said Mr. Trump was not paid for taking the trip. But his airfare, and that of his girlfriend, Bettina Anderson, was covered by Mr. Parscale, who has a business partner based in Serbia. Mr. Parscale declined to comment or to disclose the name of his Serbian business partner.

Virginia Canter, a former ethics adviser to the International Monetary Fund, said that Donald Trump Jr.’s meeting with the Serbian president was reminiscent of activity by Hunter Biden, who was accused by Republicans of leveraging the position of his father, Joseph R. Biden Jr., as vice president to make lucrative overseas business deals.

“It is kind of the height of hypocrisy that they were concerned about Hunter Biden’s foreign work,” said Ms. Canter, who also served as an ethics lawyer in the Clinton White House and now works at a nonprofit group called State Democracy Defenders Action, which has been critical of Mr. Trump.

In Ms. Canter’s view, the conflict of interest in Donald Trump Jr.’s case is more explicit.

“Don Jr., as a surrogate for his father, is using the public office of the president of the United States to help the president of Serbia stay in office — while furthering the Trump family’s personal financial interest,” she said. “It is unethical. It’s offensive.”

It remains unclear how much Mr. Trump’s presence in Serbia may have helped Mr. Vucic.

Several days after the visit, the streets of central Belgrade were jammed with more than 100,000 demonstrators for what organizers called one of the largest protests in the nation’s history.

Mr. Vucic’s government offered the Trump family a deal last year, as President Trump was running for re-election, to gain access to the prime real-estate development site in the middle of Belgrade.

The government is leasing the site to Mr. Kushner’s real-estate partnership for 99 years, according to Serbian officials. Affinity Global Development, the Kushner affiliate, in return has agreed to build the hotel and luxury apartments in a partnership with Mohamed Alabbar, a business executive from the United Arab Emirates.

Donald J. Trump, before he was first elected president and while he was still running the family real-estate business, had first considered building a hotel at this exact site in 2013 and associates of the Trump Organization traveled to Belgrade to inspect the location. The project did not come together before Mr. Trump’s election in 2016, but Mr. Kushner revived it last year while Mr. Trump was running again for office.

The hotel project had generated smaller scale protests in Belgrade even before the fatal rail station canopy collapse late last year.

Opposition leaders like Mr. Jonic argued that the former Ministry of Defense site was symbolic because it was attacked by NATO forces led by the United States in 1999 when Serbia and its neighbor Montenegro were part of Yugoslavia. It should not be turned over to American real-estate developers seeking a profit, the opposition leaders said.

“Can you imagine an American president, any president, giving West Point as a gift to an offshore company, only to demolish it and build a hotel?” Aleksandar Jovanovic, a member of Serbia’s parliament, said last year as the deal was being negotiated, referencing the U.S. Military Academy.

“One would have to have a vivid imagination to imagine that. Unfortunately, what is unthinkable in America is a tragic reality in Serbia,” he said at that time.

Donald Trump Jr., in addition to being shown the layout of downtown Belgrade by Serbia’s president, conducted a nearly hourlong interview with Mr. Vucic that was broadcast in recent days on Mr. Trump’s podcast, “Triggered.”

During the conversation, Mr. Trump compared the protests in response to the November rail station collapse to criticism of the Jan. 6, 2021, attack by his father’s supporters on the Capitol in Washington.

“It was later weaponized,” Mr. Trump said during the interview, before continuing with theories raised by Trump allies related to events in Washington “like our, you know, Jan. 6 turned into something that it wasn’t, to incite potentially even a revolution.”

Mr. Trump and Mr. Vucic also talked about Russia and the war in Ukraine and Mr. Vucic’s work with President Trump during his first term.

They both asserted separately that funding from the U.S. Agency for International Development, which the Trump administration has slashed over the last two months, had been improperly used by some nonprofit groups in Serbia to play a role in the protests, although neither offered proof of this allegation.

The Trump family’s evident support of Mr. Vucic is much appreciated, the Serbian president made clear, adding that he believes it is part of the reason President Trump is so popular in Serbia.

“This was the country where Trump was enjoying the biggest popularity in the entire Europe by far,” Mr. Vucic said. “I’m not flattering him or I’m not flattering you. I’m saying what people here think.”

Andrew Higgins contributed reporting.