The court "refused the request to arrest Alexei Navalny" for allegedly using the Internet in breach of his house arrest conditions, said his spokeswoman Kira Yarmish.
"For us it is clear that this is a politically motivated case from start to finish," she added.
One of the most vocal critics of President Vladimir Putin, Navalny faces charges of stealing and laundering USD 760,000 from French cosmetics company Yves Rocher. He has been under house arrest since February and banned from using the Internet.
"Prosecutors have evidence that Navalny has used the Internet," Nikolai Lyaskin told AFP from the court earlier Friday.
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A spokesman for the prison service declined to comment.
Navalny's supporters fear the trial could result in a lengthy prison term, a move that would deliver a huge blow to Russia's already demoralised opposition.
Last July, the activist was convicted of embezzlement and sentenced to five years in jail in a separate case but walked free the following day in a surprise move that allowed him to run a high-profile campaign for Moscow mayor.
In a post on his supporter-maintained blog, Navalny remained defiant, saying this second threat to toss him back into jail hadn't scared him.