Tom Barrack, a real estate investor and close ally of former President Donald Trump, was detained on allegations of acting as a foreign agent of the United Arab Emirates and lying to federal authorities on Tuesday in Los Angeles.

Mr. Barrack, along with a former employee and an Emirati businessman, allegedly acted on the orders of senior Emirati officials to influence Mr. Trump's policy positions and to sway public opinion in favor of UAE interests, according to a seven-count indictment filed by the US Attorney's office in Brooklyn, N.Y.

Mr. Barrack, who is 74 years old, is charged with one count of conspiracy and one count of working as an unregistered agent of a foreign government. During a June 2019 interview with federal law enforcement authorities, he was also charged with obstruction of justice and four counts of providing false statements.

Mr. Barrack was a staunch Trump supporter who served as an informal Middle East adviser during the 2016 campaign and later ran unsuccessfully for the position of special envoy to the Middle East, according to the indictment. He went on to chair President Donald Trump's inauguration committee in 2017, which was the subject of two federal investigations investigating its spending and possible illicit foreign donations.

A magistrate court ordered him temporarily held at a hearing in Los Angeles on Tuesday afternoon, awaiting a second hearing next week. He didn't plead guilty.

Mr. Barrack's spokesperson stated, "Mr. Barrack has made himself voluntarily available to investigators from the beginning." He is not guilty, and he will enter a not guilty plea.”

Mr. Barrack stepped down as executive chairman of Colony Capital Inc., the investment firm he founded in 1991, earlier this year. Colony changed its name to DigitalBridge last month.

The indictment details the UAE's efforts to build strong ties with the Trump campaign and then the Trump administration, all while Persian Gulf Arab countries tried to persuade Washington to support their Middle East agenda, which included abandoning the Iran nuclear deal and cracking down on the Muslim Brotherhood. Prosecutors claim Mr. Barrack collaborated with his two co-conspirators on a "wish list" of policy accomplishments the UAE desired during the Trump administration.

Yousef Al Otaiba, the Emirati ambassador to the United States, did not respond to a request for comment.

According to the indictment, Mr. Barrack and the other charged persons exchanged detailed messages as they planned speeches and policy platforms, discussed potential Trump administration hires, and communicated direction from Emirati officials.

According to the indictment, in the spring of 2016, Mr. Barrack presented himself to top Emirati officials as a major channel to Mr. Trump, with whom he had a decades-long friendship, with the support of Emirati billionaire Rashid Al-Malik.

Mr. Malik was also charged with conspiracy and working as a foreign government agent in the indictment. According to a court filing by prosecutors on Tuesday, he is still on the loose: Mr. Malik fled the United States three days after an April 2018 interview with law officials.

Prosecutors say Mr. Barrack collaborated with Mr. Malik to draft text for then-candidate Mr. Trump's May 2016 address on energy policy, which included consultation with senior UAE officials. According to the indictment, Mr. Malik texted Mr. Barrack a proposal for language complimenting one Emirati official by name, stating, "This is what I got from them."

Prosecutors said that language was added to a circulated draft but was ultimately removed from Mr. Trump's speech. According to the indictment, another Emirati official told Mr. Barrack that UAE officials were "happy with the results."

According to the indictment, Mr. Barrack promoted the UAE and its interests in media appearances while working with Mr. Trump's campaign and inauguration committee, after receiving guidance from Mr. Malik and Emirati officials.

Mr. Malik informed Matthew Grimes—the former Colony employee charged in the case—that the Emirati officials didn't like the term "dictatorships" and "don't want to be labeled as dictators" during the drafting of an opinion article by Mr. Barrack in October 2016, according to the indictment. According to the accusation, Mr. Grimes consented to modify the terminology to "regimes."

According to the indictment, Messrs. Malik, Barrack, and Grimes began working with Emirati officials in June 2016 to develop a strategic approach to strengthen UAE influence in the United States. According to the indictment, a draft presentation explaining the approach proposed a "guidance board" to direct UAE investments that were "intertwined" with the country's foreign policy and economic aims.

Acting as an unregistered agent and conspiring to act as an unregistered agent are charges against Mr. Grimes. Mr. Grimes was ordered detained pending a hearing next week by a magistrate judge in California, citing his danger of fleeing.

Mr. Barrack's friendship with President Trump predates his election. In 1988, he engineered the sale of New York's Plaza Hotel to Mr. Trump, and he attempted to purchase Trump Hotels & Casino Resorts Inc. from Mr. Trump on multiple occasions.

Mr. Barrack became a crucial role in Mr. Trump's first presidential campaign, helping him raise more than $30 million by June 2016.

Mr. Barrack gained control of the incoming president's inauguration committee after Mr. Trump won the White House. Federal authorities are looking into how the group earned and spent more than $100 million, more than twice what President Barack Obama raised for his inauguration in 2009.

The investigation, which was overseen by the United States Attorney's Office in Manhattan, did not result in any criminal charges being filed against members of the inauguration committee. According to persons familiar with the situation, the US Attorney's office in Brooklyn was also looking into the inauguration committee's illicit foreign donations.

Mr. Trump claimed in an interview with The Wall Street Journal that as president, he contemplated appointing Mr. Barrack for several positions, including US ambassador to Mexico. That nomination, however, never materialized. Mr. Barrack instead played a supporting role in the administration's foreign strategy.

Mr. Barrack's ties to the Middle East, as well as the blurring of borders between his commercial interests and White House influence, have already been questioned.

Mr. Barrack sought Saudi government finance in an attempt to buy faltering U.S. nuclear-reactor producer Westinghouse, according to documents revealed by the House Oversight Committee in 2019. He also advocated for and received Trump administration backing for the approach. Mr. Barrack advocated for the Saudi nuclear proposal while also seeking a key White House job dealing with Middle East policy, according to the committee's report.