EU foreign policy chief Josep Borrell heads to Moscow on Thursday to press the Kremlin to free critic Alexei Navalny, who faces a court hearing next week that could hand him a hefty jail term.   

Borrell's three-day trip to Russia -- the first for a top envoy from Brussels since 2017 -- has drawn criticism from some of the bloc's 27 nations worried it will send the wrong signal to Moscow. 

But the former Spanish minister has insisted he will pass on a "clear message" to his counterpart Sergei Lavrov when he meets him on Friday.

"The relationship with Russia is one of the EU's most complex. Recent developments only serve to further underline the need for me to visit Moscow," Borrell said in a statement Friday detailing the visit. 

"But beyond the issues of contention there are also areas in which the EU and Russia do cooperate, or need to cooperate more, that require our urgent attention."

Tensions are high after Navalny was arrested this month as he returned to Moscow from Germany, where he was recovering from a near-fatal poisoning with a Novichok nerve agent. 

That move sparked widespread protests across Russia on Saturday that saw thousands of demonstrators detained by baton-wielding riot police. 

A Moscow court will begin a hearing on Tuesday against Navalny that could see an earlier 3.5-year suspended sentence converted into prison time. 

Some EU states have mooted sanctions over the crackdown but so far the bloc has held off as it waits to see if President Vladimir Putin's most prominent domestic opponent is released. 

The EU in October slapped asset freezes and visa bans on six senior Russian officials over the "use of chemical weapons in the assassination attempt" of Navalny.

Relations with Moscow nosedived in 2014 over its annexation of Crimea and fuelling of the conflict in east Ukraine.

Borrell -- who is also set to meet representatives of Russian civil society -- is looking to enlist the Kremlin's help in efforts to revive the 2015 nuclear pact with Iran. 

He will be keen for Moscow to exert its influence over Tehran as new US President Joe Biden weighs rejoining the deal.