The Foreign Secretary said on Twitter that Navalny should be “immediately released” after he was detained at a Moscow airport on Sunday after spending five months in Germany recovering from nerve agent poisoning.
“It is appalling that Alexey Navalny, the victim of a despicable crime, has been detained by Russian authorities. He must be immediately released,” Mr Raab said.
“Rather than persecuting Mr Navalny, Russia should explain how a chemical weapon came to be used on Russian soil.”
Mr Navalny’s detention has been denounced by governments around the world, with the European Union, Germany and the US all speaking out about the arrest.
In a statement, French foreign ministry spokeswoman Agnes von der Muhll said: “France takes note with great concern of the arrest in Russia of Mr Alexei Navalny. With its European partners, it is following the situation with the utmost vigilance and call for his immediate release.”
US secretary of state Mike Pompeo said he was “deeply troubled” by Mr Navalny’s arrest and criticised Russian President Vladimir Putin.
“Confident political leaders do not fear competing voices, nor see the need to commit violence against or wrongfully detain political opponents,” he said.
Labour shadow foreign secretary Lisa Nandy also said Mr Navalny’s arrest was “unjustifiable and an insult to the Russian people”.
She said: “Alexei Navalny was the victim of a cowardly chemical weapons attack and has shown great courage in returning to his homeland to continue to champion democratic reform.
“His detention is unjustifiable and an insult to the Russian people.
“Navalny should be released immediately and those responsible for the chemical attack that almost cost him his life brought to justice.
“We stand in solidarity with the Russian people, who have the right to exercise their democratic rights and determine the future of their country.”
Prior to Mr Navalny’s arrival at Moscow’s Sheremetyevo airport, Russia’s prisons service said the 44-year-old had violated parole terms from a suspended sentence on a 2014 embezzlement conviction.
Officials said he would be held in custody until a court rules on his case.
Tom Tugendhat, the foreign affairs select committee chairman, said Russia’s justification of Mr Navalny’s arrest was “designed to deceive”.
“Navalny missed probation hearings because he was recovering after the same Putin officials who ordered his arrest tried to murder him,” the Conservative MP tweeted.
“If he gets beaten up and there are marks on the weapons, they’ll charge him with damaging Government property.”
Mr Navalny fell ill in August on a flight from Siberia to Moscow. The 44-year-old was later flown to Berlin and kept in an induced coma for two weeks.
A German military lab has determined Mr Navalny was poisoned with Novichok, the same class of Soviet-era agent the UK said was used on former Russian spy Sergei Skripal and his daughter in Salisbury in 2018.
On Monday, the German government said independent tests by labs in France and Sweden backed up its findings.
Members of the Kremlin have denied any involvement and Russian investigators have said there are no grounds to launch a probe.