Jonathan Spaulding, 32, of Wayland, Massachusetts, said he was seized by three men on the platform of the Prospekt Vernadskogo metro station shortly before 5 p.m.
He said the men, who were dressed in leather jackets and looked like "thugs," led him to the police office at the metro, where they were joined by a uniformed officer.
The men threw him behind bars and then one of them kicked him in the legs, stomach and chest and beat his head, he said.
"He basically told me he was going to kill me," said Spaulding.
When he asked them for identification, the three men flashed, but did not open, small red booklets with gold letters bearing police force insignia.
Lieutenant Sergei Yermolin, the duty officer for the metro's red line, said the men who seized Spaulding were policemen from Krasnodar studying at a Moscow academy and living at the Hotel Kometa.
He said they observed Spaulding on the metro and thought he might have been pickpocketing.
They said when they followed Spaulding off the metro and asked him to follow them, he resisted violently.
"He kicked and hit them," Yermolin said, "I don't know about in your country, but in our country, they give you 15 days for that."
Spaulding said he passively resisted arrest by going limp and making it more difficult for the men to carry him, something he learned in civil disobedience training for his protest work. The three men left, Spaulding said, after he showed them his U.S. passport and a document showing that he once worked at the U.S. Embassy in Kiev.
Yermolin denied that Spaulding was beaten, stating that the officer on duty at the metro station at the time, Sergeant Nikolai Belsky, was an Afghan war veteran and would not let a thing like that happen. It was Belsky, he said, who ordered Spaulding released from custody.
In the event that Spaulding was hurt, Yermolin said he could appeal to Moscow police headquarters.
"If that's the case, let them apologize," he said.
Just before the incident, Spaulding had given the second of two television interviews in which he criticized the Moscow police for their treatment of environmental protesters.
Yermolin said he had no idea Spaulding was an environmental activist.
The U.S. Embassy had no immediate comment on the case, although Spaulding said he did report the incident to the embassy's office of American Citizens' Services.
Spaulding arrived in Moscow recently after participating in the Walk Across Europe for a Nuclear Free World.
He was one of several activists arrested in front of the French Embassy Oct. 3 while protesting France's nuclear tests in the South Pacific.
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