Regime critic Alexei Navalny was arrested immediately after landing at Vnukovo airport outside Moscow on Sunday.
Several top European politicians are now sharply criticizing Russia's actions.
"I am very worried about the news that Alexei Navalny has been arrested on his arrival in Moscow.
Calls on Russian authorities to release Navalny.
An active and vigorous opposition should be welcomed ", tweets Foreign Minister Ann Linde (S).
The same message can be found in a tweet from European Council President Cahrles Michel.
"The arrest of Alexei Navalny in connection with his arrival in Moscow is unacceptable.
I urge Russian authorities to release him with immediate effect, "he wrote.
The demand for a speedy release also comes from Estonia, Latvia, Lithuania, which in a joint statement demand that the EU impose sanctions on Russia if this does not happen.
The sanction demand is backed by Poland, Iceland and Austria, among others.