Russian opposition campaigner Ruslan Shaveddinov has been forced into the military and posted to a remote air base in the Arctic.
However, the authorities insist he had been trying to dodge compulsory military service.
Alexei Navalny said Mr Shaveddinov, a project manager at his Anti-Corruption Foundation, was detained at his Moscow flat on Monday after the door was broken down, the electricity cut, and the SIM card on his mobile phone remotely disabled.
Shaveddinov resurfaced at a remote military base on Novaya Zemlya, a freezing archipelago in the Arctic Ocean some 2,000 km (1240 miles) north of Moscow and the location of a missile air defence unit.
Mr Navalny was also himself detained after a raid on his offices on Thursday, but he has now been released.
He is a fierce critic of Russia's President Vladimir Putin and state officials.
Mr Navalny said Shaveddinov, who had earlier tried to appeal against his conscription on medical grounds, managed to make one phone call on Wednesday using someone else's phone.
He said he had been told he would not be allowed to have a mobile phone during his one year of military service and said another soldier had been assigned to accompany him at all times to watch what he was doing.
Navalny said lawyers for Shaveddinov would be challenging his conscription and would argue he had been illegally kidnapped and imprisoned.
"Serving in the army has simply turned into a way of locking people up," Navalny wrote on social media.
Opposition activists likened MR Shaveddinov's treatment to the way in which Tsarist Russia and the former Soviet Union sent political opponents to far-off corners of what is the world's largest country by territory.
Mr Shaveddinov was part of Navalny's unsuccessful campaign to run against Vladimir Putin for the presidency in 2018.
He worked as a TV presenter for Navalny's online channel, and helped manage projects at Navalny's foundation which specialises in publishing corruption investigations into state officials and managers.
But Colonel Maxim Loktev, the deputy military commissar for Moscow, told the TASS news agency that Shaveddinov had dodged mandatory conscription for a long time and that a court on Monday had ruled his conscription legal.
One year's military service is mandatory in Russia for all male citizens aged 18-27, with some narrow exceptions.
Kremlin spokesman Dmitry Peskov said Shaveddinov's treatment looked legal if he'd been a draft dodger.
"If he evaded conscription, he broke the relevant law of the Russian Federation," said Mr Peskov. "If he dodged conscription and was conscripted in this way then everything was done strictly in accordance with the law."