Russia's state Investigative Committee said it had opened a murder investigation. State-owned RIA news agency said 25 people were wounded and 19 of them were being treated in hospital.
A leading Russian official pointed the finger at Ukraine, without providing evidence. A Ukrainian presidential adviser said "domestic terrorism" was breaking out in Russia.
Tatarsky, whose real name was Maxim Fomin, had more than 560,000 followers on Telegram and was one of the most prominent of the influential military bloggers who have championed Russia's war effort in Ukraine while often criticising the army top brass.
"We'll defeat everyone, we'll kill everyone, we'll rob everyone we need to. Everything will be as we like it," he was shown saying in a video clip last September at a Kremlin ceremony where President Vladimir Putin claimed four occupied regions of Ukraine as Russian territory - a move rejected as illegal by most countries.
TASS news agency quoted an unnamed source as saying the bomb was hidden in a miniature statue that was handed to Tatarsky as he addressed a group of people in the cafe.
Mash, a Telegram channel with links to Russian law enforcement, posted a video that appeared to show Tatarsky, microphone in hand, being presented with a statuette of a helmeted soldier. It said the explosion happened minutes later.
"He was killed vilely. Terrorists cannot do otherwise. The Kyiv regime is a terrorist regime. It needs to be destroyed, there's no other way to stop it," he said.
'RIPE ABSCESS'
Mykhailo Podolyak, a Ukrainian presidential adviser, wrote on Twitter that it had only been a matter of time - "like the bursting of a ripe abscess" - before Russia would be consumed by what he called domestic terrorism.
"The spiders are eating each other in a jar," he said.
Tatarsky's death followed the killing last August of Darya Dugina, the daughter of a prominent ultra-nationalist, in a car bomb attack near Moscow.
Russia's Federal Security Service accused Ukraine's secret services of carrying out that attack, which Putin called "evil". Ukraine denied involvement.
Russia's war bloggers, an assortment of military correspondents and freelance commentators with army backgrounds, have enjoyed broad freedom from the Kremlin to publish hard-hitting views on the war, now in its 14th month. Putin even made one of them a member of his human rights council last year.
They reacted with shock to the news of Tatarsky's death.
"He was in the hottest spots of the special military operation and he always came out alive. But the war found him in a Petersburg cafe," said Semyon Pegov, who blogs under the name War Gonzo.
Alexander Khodakovsky, a leading pro-Moscow figure in eastern Ukraine, wrote: "Max, if you were a nobody, you'd have died of 'vodka and headcolds'. But you were dangerous to them, you did your business like no one else could. We will pray for you, brother."
