A U.S. citizen is among Navalny supporters detained in Russia.

Law enforcement agents in Russia have detained at least three supporters of the jailed opposition leader Aleksei A. Navalny, including a Russian-born American citizen, on charges of backing an extremist organization.

Details of the case emerged earlier this week in Russian news media reports, and a State Department spokesman on Friday confirmed the arrest but declined to provide other details.

The American citizen, Ilya A. Startsev, 37, was among those arrested in various cities in Russia on charges of donating money to Navalny’s Anti-Corruption Foundation, according to OVD-Info, a rights group that reports on repression in Russia. The foundation, known by its Russian initials F.B.K., has regularly embarrassed senior Russian officials, including President Vladimir V. Putin, by exposing vast holdings they have allegedly accumulated while in office, far beyond what their government paychecks would allow.

The government disbanded the Anti-Corruption Foundation in Russia in 2021 by declaring it an extremist organization, with the group’s main investigators fleeing into exile, where they continue to work. Various Navalny supporters have faced criminal charges in Russia as the Kremlin has ratcheted up repressive measures, especially trying to silence critics of its war in Ukraine.

A conviction for financing an extremist organization carries an eight-year jail sentence.

Mr. Startsev was born in Russia but moved to the United States as a child after his mother married an American, according to an interview he gave a few years ago, as well as accounts in Russian news media. After moving to the Chicago area, he attended high school and Northern Illinois University, according to his online biography.

He moved back to Russia to pursue a teaching career. At the time of his arrest, he was teaching English for a Moscow company called the American Club of Education, which offers both online and in-person courses. He had also taught at a private school in Oryol, a provincial city more than 220 miles southwest of Moscow. He was jailed there on Thursday, a day after being detained in Moscow, Russian news reports said.

In recent years there have been several cases of Americans being arrested on unlikely spying charges in Russia. Evan Gershkovich, a reporter for The Wall Street Journal, is incarcerated in Moscow facing trial, and Paul Whelan, 53, a former Marine, is serving a 16-year sentence.

On social media, Mr. Startsev made no secret of his staunch support for Mr. Navalny, who survived what is widely considered an attempt by government agents to kill him with poison, and has since been jailed in a Russian penal colony on various charges that have extended his sentence to at least 19 years.

In his last writing on VKontakte, the Russian equivalent of Facebook, Mr. Startsev lauded Mr. Navalny’s aims, and supported his vision of a “free, happy and uncorrupt future” for Russia, he wrote, with virtuous politicians being a key to this.

He had also posted a picture of himself on VKontakte, holding up a poster of what appears to be an opposition rally in May 2018. The hand-lettered poster called for fighting corruption, poverty and inequality; restoring free elections; giving the opposition access to the media; releasing political prisoners; ending government control of the internet and allowing the freedom of assembly.

In his 2019 interview, Mr. Startsev said he had used his time in the United States to improve his credentials in order to get a better job back in Russia. Asked about life in the United States, he said, “The food there has a different taste; the air is cleaner; the roads are better.”

He had learned a lot, he said, but he wanted to apply it in Russia: “I want to help Russia become a great and beautiful country.”

Milana Mazaeva contributed reporting.