A top Ukrainian official suggested internal struggles over the Kremlin's invasion of Ukraine were behind the blast.
An explosion at a cafe in St. Petersburg has killed one person and injured at least 30 more.
One victim has been identified as a prominent Russian military blogger who supported President Vladimir Putin's invasion of Ukraine.
Russian news reports said Vladlen Tatarsky was killed in the explosion at the Street Food Bar No. 1 cafe on Sunday during an event organised by Russian nationalists. He was a guest speaker when the bomb went off.
10 people were in a critical condition in hospital, according to Russia's RIA state news agency.
It is not immediately who was behind the attack.
However, Darya Tryopova, a St. Petersburg woman, has been arrested on suspicion of being involved in the bombing, Interfax reports. It said that she had been previously detained for taking part in anti-war rallies.
According to Russian media and other military bloggers, a woman presented him with a box containing a statuette that apparently exploded. The organisers of the event are calling it a "terrorist attack".
This is the second seeming assassination of a figure linked to the Ukraine war on Russian soil.
Last August, Darya Dugina, a 29-year-old commentator for a nationalist Russian TV channel, died when a remotely controlled explosive device planted in her SUV blew up.
She was the daughter of Alexander Dugin, an ultra-nationalist philosopher, widely considered Putin's brain. Some suggested he was the intended target.
Russian authorities blamed Ukraine for the attack, but Kyiv denied involvement.
Tatarsky was born in Donetsk, southeastern Ukraine. He had reported from the Ukrainian frontline and gained particular notoriety last year after posting a video filmed inside the Kremlin in which he said: "We will defeat everyone, we will kill everyone, we will rob everyone as necessary. Just as we like it."
He had more than 560,000 followers on Telegram.
Though a staunch supporter of the invasion, Tatarsky had publicly criticised the Russian military command over their operation.
The cafe targeted on Sunday was previously owned by Yevgeny Prigozhin, the head of Russia's notorious Wagner mercenary group, a St Petersburg news site reported.
A group called Cyber Front Z, calling itself "Russia's information troops" on Telegram, said it had hired out the cafe for the evening.
"There was a terrorist attack. We took certain security measures but unfortunately, they were not enough," it posted on the social media platform.
Tatarsky had deep connections with the Wagner Group, the Kremlin and Donetsk People's Republic.
He was a well-known figure in Russia's influential military blogosphere, posting videos analysing the situation on the ground since the start of the Ukraine offensive.
In 2014, he fought for several months with pro-Russian militants in eastern Ukraine, according to a local news outlet Fontanka.
On Sunday, the Russian Foreign Ministry paid tribute to a "defender of the truth".
"Russian journalists constantly feel the threat of reprisals from the Kyiv regime," said ministry spokeswoman Maria Zakharova. "It is thanks to Russian war correspondents that the world sees the real images of the operations and discovers what is happening in Ukraine".
Adviser to the Ukrainian presidency Mykhaïlo Podoliak reacted quickly, calling the incident "internal terrorism".
That it would become "an instrument of internal political struggle was only a matter of time", he added.