The Russian Foreign Ministry launched an online campaign over the weekend in support of Maria Butina, who was arrested in the United States on espionage charges.
Butina, 29, was arrested on July 18 and charged with conspiracy to act as an agent of the Russian government, which the ministry described as "false accusations".
Last week, the U.S. grand jury returned an indictment against Butina, a recent graduate of American University in Washington and founder of a pro-gun Russian advocacy group, and added a charge accusing her of acting as a Russian government agent while developing ties with U.S. citizens and infiltrating political groups.
The latter charge carries a maximum penalty of 10 years in prison. The conspiracy charge carries a maximum five-year prison term.
Russian Foreign Minister Sergei Lavrov told his U.S. counterpart Mike Pompeo on Saturday that the arrest of Butina in the U.S. was "unacceptable", AFP reported.
In a telephone call with Pompeo, Lavrov "stressed that the actions of American authorities who arrested the Russian citizen Butina based on false accusations is totally unacceptable", the Foreign Ministry said in a statement.
Lavrov said Butina must be released "as soon as possible", according to the statement.
Butina's arrest added to the political turmoil in Washington over Moscow's alleged meddling in the 2016 presidential election and an investigation into whether President Donald Trump's campaign collaborated with Russians.
The arrest was announced hours after Trump finished a summit and a news conference with Russian President Vladimir Putin in Helsinki, Finland on July 16.
"All she did was study, working very hard on a daily basis. Naturally, the pressure was high because English is not her native language, full of unfamiliar terms. What I'm saying is that she was studying almost round-the-clock," Valery Butin, Butina's father, told Rossiya-1 television.
He added that the last time he spoke with Maria was shortly before she was arrested.
"She had to meet and contact many people just as her future profession calls for. Well, it looks like some people in some places circles got interested. She never did anything illegal, I can vouch for that," Butin said.
Robert Driscoll, an attorney for Butina, said she was not a Russian agent.
However, the U.S. Justice Department said Butina is accused of operating at the direction of a high-level official of the Russian Central Bank who was recently sanctioned by the U.S. Treasury.
The department also said in its complaint that Butina worked with two unnamed U.S. citizens and the Russian official to try to influence U.S. politics and infiltrate a pro-gun rights organization.
The complaint did not name the group.
The Russian Foreign Ministry also launched a Twitter flashmob late last week calling for followers to change their profile pictures to an image of Butina. The ministry's official Facebook account profile picture has changed to a photo of Butina.
Foreign Ministry spokeswoman Maria Zakharova said the timing of Butina's detention was a clear attempt to minimize the positive outcome of the summit.
"It seems that the U.S. FBI, instead of dealing with its basic duties to combat crime, is carrying out a plainly political order," Zakharova said.