The Russian authorities blamed Ukraine and Russian opposition activists on Monday for the bombing that killed a popular pro-war blogger a day earlier, signaling that the Kremlin could use the dramatic attack in St. Petersburg to escalate its already harsh crackdown against what remains of antiwar activism in Russia.
The Russian government’s Antiterrorism Committee issued a statement claiming, without providing evidence, that the deadly bombing at a pro-war gathering at a St. Petersburg cafe was planned by Ukrainian intelligence agencies, along with “agents” connected to the movement of the imprisoned opposition leader Aleksei A. Navalny.
The police detained a Russian woman on Monday and released a video that it said showed her confessing that she had delivered a statuette containing a bomb to the blogger, who was known as Vladlen Tatarsky. The Antiterrorism Committee claimed that she was an “active supporter” of Mr. Navalny. An exiled leader of Mr. Navalny’s movement, Ivan Zhdanov, described the accusations against his team as outrageous and said they were a pretext to extend Mr. Navalny’s prison term even further.
Dmitri S. Peskov, the spokesman for President Vladimir V. Putin, sounded slightly less definitive in his daily briefing with journalists, cautioning that the investigation into the bombing was continuing. He said that “Ukrainian intelligence agencies may have had something to do with the planning of this terrorist attack.”
Still, the fast-moving developments on Monday indicated that the Kremlin was preparing to use the attack — in the heart of Russia’s second-largest city — to try to even further ostracize domestic opponents of Russia’s invasion of Ukraine and of Mr. Putin. Russian news media reports described the woman arrested after the attack, Daria Trepova, as an opponent of the war, citing interviews with her friends.
Tatiana Stanovaya, an analyst for the Carnegie Endowment for International Peace, said that the assertions would further divide Russian society.
“All participants in antiwar actions will now automatically become potential terrorists in the eyes of not just law enforcement, but also the ‘patriotic’ public,” she wrote.
The explosion that killed Mr. Tatarsky, whose real name was Maksim Fomin, was the most brazen attack on a prominent war supporter inside Russia since the car bombing in August that killed Daria Dugina, the daughter of an ultranationalist ideologue, Aleksandr Dugin.
It came amid escalating drone attacks deep inside Russian territory and shelling and deadly raids on the regions bordering Ukraine, violence that has begun to expose residents of major Russian cities to fallout from a war that the Kremlin has sought to portray as a distant “special military operation.”
Ms. Trepova, a 26-year-old native of St. Petersburg, had appeared on the national police’s wanted list earlier on Monday, hours before Russia’s investigative police force said in a brief statement that she had been arrested in connection with the St. Petersburg bombing. Court records show that a woman with the same name and birth date received a 10-day jail term last year for participating in a protest on the first day of Russia’s invasion of Ukraine.
She was not, however, widely known in Russian opposition circles, and her social media profiles are largely free from explicit political content. The Russian antiterrorism agency did not offer evidence for its claim that Ms. Trepova was working with Ukrainian intelligence, and the Ukrainian government has not commented on the attack.
Mr. Tatarsky — who took his pen name from the hero of a cult novel about dissolute post-Soviet life — was giving a talk on his trips to the front line in Ukraine when a bomb exploded in the cafe where he was speaking.
The venue, called Street Food Bar #1 Cafe, is owned by the head of Russia’s Wagner mercenary group, Yevgeny V. Prigozhin, who said he had allowed it to be used by a nationalist activist group that organized the event with Mr. Tatarsky. Mr. Prigozhin said he did not believe the Ukrainian government was behind the attack, saying, “This is an act of a group of radicals that is unlikely to have connection to the government.”
Videos posted on social media showed Mr. Tatarsky receiving a small statue in his likeness onstage shortly before the explosion.
“What a beautiful guy, is that me?” Mr. Tatarsky asked the audience after receiving the statue, according to one video. The authenticity of the video could not be immediately verified.
After the arrest was announced on Monday, Russia’s Interior Ministry posted a short video showing a woman it said was Ms. Trepova telling interrogators that she had given Mr. Tatarsky the statue, adding that she had received it from a person she declined to name.
Ivan Nechepurenko and Oleg Matsnev contributed reporting.