The EU's managing director for Eastern Europe, Russia and Central Asia, Michael Siebert, summoned Kirill Logvinov and "called upon Russia to allow an independent and transparent international investigation into circumstances of Alexei Navalny's sudden death", said a statement from EU's diplomatic service.

"The EU side conveyed EU's outrage over the death of the Russian opposition politician Alexei Navalny, for which the ultimate responsibility lies with President Putin and the Russian authorities."

It "urged Russia to release his body to his family without further delay and allow the family to organise a funeral".

"Russia must immediately and unconditionally release all other political prisoners, as well as all those detained in cities across Russia when paying tribute to Alexei Navalny", it said.

Navalny, 47, died in a remote Arctic prison last Friday, according to Russian prison authorities.

His mother, Lyudmila Navalnaya, travelled to the remote IK-3 penal colony on Saturday, the morning after his death was announced, and has since been barred from seeing his body.

Russian investigators said the body would be kept for "at least two weeks", according to Navalny's team, which says the charismatic leader was murdered.

Belgium, France, Germany and Poland are among the countries that have summoned Russian ambassadors over Navalny's death.

The United States is due to announce fresh sanctions on Russia on Friday over Navalny's death.