The Government has said it is “deeply concerned” by the arrest of Russian opposition leader Alexei Navalny.
Mr Navalny was detained at a Moscow airport on Sunday after spending five months in Germany recovering from nerve agent poisoning that he blames on the Kremlin.
Prior to his arrival at Moscow’s Sheremetyevo airport, Russia’s prisons service said the 44-year-old has violated parole terms from a suspended sentence on a 2014 embezzlement conviction.
Mr Navalny’s detention has been condemned by governments around the world.
A statement from UK’s Foreign, Commonwealth & Development Office (FCDO) said: “We are deeply concerned by the detention on 17 January of Alexei Navalny.
“Instead of persecuting the victim of this terrible crime, the Russian authorities should investigate how a chemical weapon came to be used on Russian soil.”
European Parliament president David Sassoli called for Mr Navalny’s immediate release.
He wrote on Twitter: “The arrest of Alexey #Navalny in Moscow is an offence to the international community, to Europe that helped save his life.
“We ask the Russian authorities for his immediate release. We are ready to invite him to the @Europarl_EN.”
Germany’s foreign minister, Heiko Maas, said in a statement that Mr Navalny’s detention is “utterly incomprehensible. Rule of law & protection of civil rights, to which Russia is bound by constitution & international obligations, must apply to him. He should be immediately released.”
US secretary of state Mike Pompeo said he is “deeply troubled” by Mr Navalny’s arrest, while adding a veiled criticism of Russian President Vladimir Putin.
He wrote: “Deeply troubled by Russia’s decision to arrest Aleksey Navalny. Confident political leaders do not fear competing voices, nor see the need to commit violence against or wrongfully detain, political opponents.”
Shadow foreign secretary Lisa Nandy led the local calls for action, tweeting: “Alexei Navalny was the victim of a cowardly chemical weapons attack. He has shown great courage in returning to his homeland.
“His detention is unjustifiable and an insult to the Russian people. He must be released immediately and his attackers brought to justice.”
Labour MP Catherine West called for a co-ordinated response from the UK Government, the European Union and the incoming Joe Biden administration in the US.
She said: “The UK govt must work at speed with EU partners & the Biden team to develop a robust response following the arrest of #Navalny in Moscow.
“It cant be right that an opposition leader, whose life was almost taken in an aggressive poison attempt, is arrested on arrival.”
SNP defence spokesman Stewart McDonald was equally forceful in his condemnation of Mr Navalny’s arrest.
He wrote on Twitter: “Having failed to crush @navalny’s bravery with poison the Russian Government will now try, and fail, by putting him in prison.
“The free world must stand in unison in calling for his release. Those who continue to shill for Putin’s various outlets must surely feel shame tonight.”
Defence committee chairman Tobias Ellwood called Mr Navalny’s decision to return to Russia “incredibly brave”, in a sentiment echoed by veterans minister Johnny Mercer.
Mr Ellwood said: “Poisoned by the FSB yet he chooses to return to Russia and has now been arrested.
“Incredibly brave stand by #Navalny in the name of democracy as we head towards Russian parliamentary elections.
“I hope the FCDO is monitoring events closely.”