Russian policemen detained historian Valery Solovei after he and dozens of other protesters marched through the centre of Moscow on Saturday to show solidarity with protests in the far eastern city of Khabarovsk.
Kremlin critic Solovei, 60, worked for the elite Moscow State Institute of International Relations until last year when he left his job for what he said were "political reasons".
A video posted online showed several policemen in body armour and black face masks grabbing Solovei and escorting him to a police car in the Pushkinskaya Square in central Moscow.
The professor had posted about the planned protest, which he called "A Day of Changes", on his Facebook page and said that the event should help people to organize and act together.
Residents of Khabarovsk, which lies about 6,110 km (3,800 miles) east of Moscow, started protesting after the July 9 arrest of popular governor Sergei Furgal on murder charges that he denies.