Chancellor Angela Merkel on Monday decried the arrest in Moscow of Russian opposition leader Alexei Navalny upon his return from treatment in Germany, and demanded his release.

Merkel's spokesman Steffen Seibert told reporters that she "condemns" Navalny's detention by police which "violates the principles of the rule of law".

He said Germany called on Russia "to release Mr Navalny immediately".

Police seized Navalny, President Vladimir Putin's most prominent opponent, at a border control post at Moscow's Sheremetyevo airport on Sunday.

His arrest came less than an hour after he returned to Russia from Germany for the first time since he was poisoned with a nerve agent in August.

Seibert said Russia had arrested the "victim" of "attempted murder with chemical weapons" and "not the perpetrator", adding that Navalny had been in Germany to receive medical treatment after the attack.

"It is absolutely unacceptable to accuse Mr Navalny of a violation of his parole over this time" he spent in Germany, Seibert said.

He said that Merkel had "repeatedly" noted that Russia's treatment of Navalny "weighed on" relations between Berlin and Moscow. 

But when asked whether it would affect plans to continue work on the Nord Stream 2 pipeline to bring Russian gas to Germany, Seibert said the German government's position was "unchanged".