A senior European Union official told Russian President Vladimir Putin by telephone on Friday to release opposition politician Alexei Navalny and demanded an investigation into the Kremlin critic's poisoning last August.
Navalny was detained in Russia on Sunday after flying home from Germany for the first time since being poisoned with what tests conducted at a German military laboratory showed was a military-grade nerve agent.
"In my call with President Putin today, I reiterated (that the) EU is united in its condemnation of Alexei Navalny's detention and calls for his immediate release," European Council President Charles Michel, who chairs EU summits, wrote on Twitter.
"Russia must urgently proceed with full and transparent investigation into the assassination attempt on him."
Navalny says Putin was behind his poisoning last August, a version of events that the Kremlin rejects. The Kremlin says it has seen no evidence that Navalny was poisoned.
"The president of the European Council informed President Putin of the grave concern in the EU and its member states over recent developments," Michel's office said in an official EU statement, referring to Navalny.
EU foreign ministers are expected to debate more economic sanctions on Russian individuals on Monday in Brussels.
EU lawmakers passed a resolution on Thursday calling for the bloc to stop the completion of the Nord Stream 2 gas pipeline, which will take Russian natural gas to Europe, in response to Navalny's arrest.