Mr Navalny, 44, was seen being led away by police at passport control. Live footage from Russian broadcaster showed him talking with dark-uniformed and masked officers at Moscow's Sheremetyevo airport, then kissing his wife Yulia before walking away with them.
The couple were returning from a five-month stay in Germany, where he had been recovering from the Novichok poisoning
Navalny collapsed on an internal flight in Siberia last August, and it later emerged he had been poisoned with a Novichok nerve agent.
"The detainment of Alexey Navalny upon arrival in Moscow is unacceptable. I call on Russian authorities to immediately release him," tweeted Charles Michel, President of the European Council.