* Navalny says Ukraine war is 'criminal'
* Opposition calls for protests
* Russian police detain 46 people - protest monitor (Adds details, note strong language in paragraph 14)
TBILISI, Sept 21 (Reuters) - Russia's opposition on Wednesday called for protests against President Vladimir Putin after he ordered the mobilisation of 300,000 reservists for what Kremlin foe Alexei Navalny said was a failing criminal war.
Putin on Wednesday ordered Russia's first mobilisation since World War Two and backed a plan to annex swathes of Ukraine, warning the West he was not bluffing when he said he would be ready to use nuclear weapons to defend Russia.
Navalny, Russia's most prominent opposition leader, said Putin was sending more Russians to their death for a failing war.
"It is clear that the criminal war is getting worse, deepening, and Putin is trying to involve as many people as possible in this," Navalny said in a video message from jail recorded and published by his lawyers.
"He wants to smear hundreds of thousands of people in this blood," Navalny said.
Since the Feb. 24 invasion, Putin has cracked down on dissent and the media, with thousands arrested at anti-war protests and a new law that calls for 15-year prison sentences for those who distribute "fake news" about the military.
Russian state television casts critics as traitors who are in the pay of the West. Putin says the country is in a battle with the West over Ukraine which he says is being used by the United States and its allies in an attempt to destroy Russia.
Russia's anti-war groups called for street protests against the mobilisation order.
"This means that thousands of Russian men - our fathers, brothers and husbands - will be thrown into the meat grinder of war," the Vesna anti-war coalition said. "Now the war has come to every home and every family."
It called for Russians to take to the streets in major cities on Wednesday. Around 46 people were detained by police across Russia for protesting against the mobilisation, the independent protest monitoring group OVD-Info said.
Navalny says Putin started a pointless war that is butchering the innocent people of both Ukraine and Russia. He casts Putin's Russia as a state run by thieves and criminals and says it will ultimately be crushed by the forces of history.
The Kremlin dismisses Navalny's claims about Putin, who it says has won numerous fair elections in Russia since 2000 and remains by far the country's most popular politician. It has dismissed Navalny's claim that Putin is corrupt as nonsense.
Navalny's allies say Russian society does not support the mobilisation drive.
"It's scary to watch how a mad bastard is destroying a country live on television," said Navalny's chief of staff Leonid Volkov, who lives outside Russia.
"In military terms, mobilisation is hopelessly late and will not change anything in this lost war," he said. Russians should see it as matter of "moral duty, survival and self-preservation" to push back against the Kremlin, he added. (Reporting by Jake Cordell; Editing by Guy Faulconbridge and Angus MacSwan)