Navalny, Russia's most prominent opposition figure, said border guards had detained him as he tried to pass through passport control at Moscow's Domodedovo airport and told him he could not leave because of a ruling by the Federal Bailiffs Service.
Russians can be barred from leaving the country if they have unpaid fines or debts.
Navalny wrote in his online blog that he was certain he had no such outstanding financial obligations.
A copy of the exit ban given to Navalny by border guards which he published online did not give a detailed reason for his ban or cite a date for the original decision. It was unclear how long the ban remained in force.
Navalny was trying to fly to Frankfurt en route to the European Court of Human Rights in Strasbourg where judges are due to rule whether his numerous detentions by police in Russia have been politically motivated or not.
The 42-year-old lawyer sought to challenge Vladimir Putin at a presidential election in March, but was barred from taking part over a past conviction which he says was politically motivated. (Reporting by Tom Balmforth Editing by Andrew Osborn)
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