Aleksei Navalny's presidential campaign chief in Moscow, Nikolai Lyaskin, was held for several hours by police without explanation on October 31.
Lyaskin wrote on Twitter on October 31 that police stopped his car, ordered him to go with them, and took him to a police station.
Hours later, he tweeted that police had released him but ordered him to report to the authorities again on November 9.
According to Lyaskin, police gave no explanation of their actions.
"They will probably find out by November 9 what to charge me with," Lyaskin wrote on Twitter, suggesting that law enforcement officers receive politically motivated orders from higher authorities.
Lyaskin, a senior supporter of Navalny, was beaten with a metal pipe by unidentified attackers in September.
The suspect later claimed that Lyaskin had paid him to carry out the attack, which Lyaskin and Navalny's team vehemently deny.
It was not clear whether Lyaskin's October 31 detainment was linked to the beating.
Navalny is campaigning for a March 2018 election in which President Vladimir Putin is widely expected to seek and secure a fourth term in the Kremlin.
But election officials have said Navalny is not eligible to run because of a felony embezzlement conviction that he says was politically motivated.
Navalny, who has riled the Kremlin with reports alleging corruption in Putin's government, received 27 percent of the vote in a Moscow mayoral election in 2013.
Putin, who has ruled Russia either as president or prime minister since 2000, has not yet formally said whether he will participate in the poll.
His control over the levers of power would make his reelection a foregone conclusion.
