European Council chief Charles Michel demanded the "immediate release" of Russian opposition activist Alexei Navalny in a call with President Vladimir Putin on Friday.
"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," Michel, who chairs EU summits, wrote on Twitter.
"Russia must urgently proceed with full and transparent investigation into the assassination attempt on him."
The EU had already condemned an attempt to assassinate Navalny and his arrest on his return to Russia after treatment, but in Friday's call, Michel expressed "grave concerns" directly to Putin.
He also told Putin that he would launch a "strategic debate" on EU-Russia relations when he convenes a summit of all 27 EU leaders in March.
Kremlin critic Navalny, 44, was arrested on Sunday as he returned to Russia from Germany for the first time since he recovered from a near-fatal poisoning with the Soviet-designed Novichok nerve agent in August.
Russia's prison service said it had detained Navalny for violating the terms of a suspended sentence he was given in 2014 on fraud charges he says were politically motivated.
The Russian opposition leader 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.
An official EU statement released Friday by Michel's office said, "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.
Demonstrations planned for Saturday
Navalny's allies are planning demonstrations on Saturday in dozens of Russian cities in support.
Several close Navalny associates, including prominent activist Lyubov Sobol and his spokeswoman Kira Yarmysh, were detained on Thursday for urging Russians to join the demonstrations and they face hefty fines and short jail stints.
Russian police said Friday that "any such protests will be regarded as a threat to public order and immediately suppressed".
EU members already have several sanctions in place against Russian targets, and capitals are mulling further action after Navalny's detention.
EU leaders met by video conference late Thursday principally to tackle the coronavirus epidemic, but afterwards Michel issued a stern statement calling again for the release of Navalny and his supporters.
(FRANCE 24 with AFP, AP and REUTERS)