Daria Trepova has been detained following the death of Vladlen Tatarsky in the blast as he held a talk on Sunday, Russia's Investigative Committee said.

Trepova's partner, Dmitry Rylov, told the Russian news site SVTV News that while his wife was against the war in Ukraine, "she would never kill" and "believes his wife was framed".

The Russian Ministry of Internal Affairs has released a video of Daria Trepova which appears to show a confession.

The circumstances under which the filming was done is not known.

Around 30 people were also wounded in the explosion, with 10 of them said to be in a critical condition, according to the authorities.

Tatarsky, 40, whose real name was Maxim Fomin, had more than 560,000 followers on Telegram.

He was one of the most prominent of the influential military bloggers, who have provided a running commentary on Russia's invasion of Ukraine.

Russian news reports said the bomb was hidden in a bust of the blogger that the suspect had given to him as a gift just before the blast.

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Counter-terror officials in Russia have declared the incident a terror attack, which they claim was planned by Ukrainian special services and involved "collaborators" from the Navalny Anti-Corruption Foundation.

The non-profit group, which has investigated corruption among high-ranking Russian officials, was founded by opposition leader Alexei Navalny, who is currently serving a jail sentence in Russia for alleged fraud and contempt of court.

The National Anti-Terrorist Committee claim Trepova is an "active supporter" of the group, which has previously been declared a "foreign agent" and banned in Russia.

'She would never kill'

Trepova's partner, Dmitry Rylov, said that while his wife was against the war in Ukraine, "she would never kill".

"I believe my wife was framed. I'm pretty sure she would never have been able to do something like that on her own," he said.

Rylov, who is a member of the fringe opposition group, Libertarian Party of Russia (LPR), said: "Together with Daria we really do not support the war in Ukraine, but we believe that such actions are unacceptable.

"I am 100% sure that she would never have agreed to this if she had known."

Tatarsky was killed as he led a discussion at a cafe on the banks of the Neva River in the historic heart of St Petersburg.

In remarks recorded on video, a witness said that a woman who identified herself as Nastya asked questions and exchanged remarks with Tatarsky at the event.

The witness, Alisa Smotrova, quoted Nastya as saying she had made a bust of the blogger but that guards asked her to leave it at the door, suspecting it could be a bomb.

Nastya and Tatarsky joked and laughed. She then went to the door, grabbed the bust and presented it to Tatarsky.

He reportedly put the bust on a nearby table, and the explosion followed. Ms Smotrova described people running in panic, some hurt by shattered glass and covered in blood.

A St Petersburg website said the cafe had at one time belonged to Yevgeny Prigozhin, founder of the Wagner Group - the private army that is fighting for Russia in Ukraine.

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A leading Russian official blamed Ukraine for the attack, without providing evidence. The claim was rebuffed by Kyiv.

Mykhailo Podolyak, a Ukrainian presidential adviser, blamed Russian domestic terrorism for the blast.

"The spiders are eating each other in a jar," he added.

'Unlikely Ukraine military is behind attack'

If Tatarsky was deliberately targeted it would be the second assassination on Russian soil of a figure associated with the war in Ukraine.

Darya Dugina, the daughter of a Vladimir Putin ally, was killed last August after a suspected explosive device detonated on the Toyota Land Cruiser she was travelling in.

Tatarsky was among hundreds of attendees at a lavish Kremlin ceremony last September to proclaim Russia's annexation of four partly occupied regions of Ukraine.

"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 on that occasion.

Military analyst Sean Bell told Sky News it "looks really unlikely" that the Ukrainian military was behind the St Petersburg attack as it wasn't a military target.

Fellow pro-war Russian bloggers have paid tribute to Tatarsky.

"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.

Following the bombing, officials in Russia announced plans to tighten security at the upcoming Victory Day celebrations in May - a highly significant event in the country's calendar.