Lithuanian intelligence services on Wednesday said that the attack on Russian opposition figure Leonid Volkov, a close ally of late opposition leader Alexei Navalny, was likely "organised by Russia".
The Baltic state's president said Moscow's potential involvement should not come as a surprise, before striking a note of defiance against Russian President Vladimir Putin: "No one is afraid of you here".
Volkov, 43, was briefly admitted to hospital after he was repeatedly struck with a hammer outside his home in Lithuania's capital Vilnius late on Tuesday.
The attack came almost a month after Navalny's death in an Arctic prison, which Volkov blamed on Putin, and days before elections that look set to extend the Kremlin chief's stay in power.
Police refused to comment on what security arrangements had been made for Volkov either before or after the attack in Lithuania, which hosts many Russian and Belarusian opposition activists.
The NATO and EU member has been a staunch Ukraine supporter since Russia invaded its neighbour more than two years ago.
In a statement, the state security department said that the assault was "likely" an operation "organised and implemented by Russia, the purpose of which was to stop the implementation of Russian opposition projects".
No suspects have been identified but Lithuanian President Gitanas Nauseda said the country's services were looking into the attack and would hopefully find the perpetrators.
Speaking to Lithuanian reporters from Paris, he said there had been a recent uptick in incidents involving Soviet monuments and the desecration of the Lithuanian flag.
"All of this adds up to a whole set of issues, incidents, where it is clear that such things are planned. We should not be surprised," he said.
"As for Putin, I can only say one thing: no one is afraid of you here," he added.
Asked by AFP to clarify the word "here", the president's office said Nauseda was referring to both Lithuania and Europe.
Deputy chief of police Saulius Tamulevicius earlier said that investigators were looking into various theories, without elaborating.
"We are taking all steps to clarify the motives and causes of the crime. Several versions are being investigated," he told LRT radio.
Volkov on Telegram said that he was struck 15 times on the leg during the attack late on Tuesday evening.
Navalny spokeswoman Kira Yarmysh earlier said that "someone broke a car window and sprayed tear gas in his eyes" before hitting him with a hammer.
Volkov's wife, Anna Biryukova, on social media shared photos of her husband's injuries, which included a black eye, a red mark on his forehead and blood on his leg, which had soaked through his jeans.
A governmental source told AFP that Volkov was released from the hospital early on Wednesday after "bruises on various parts of the body and a fracture of the palmar bone were identified".
"More serious injuries were avoided," the source said.
