- Lithuanian intelligence originally said the attack was likely "Russian-organised".
- At a press conference on Friday, Lithuania's President Gitanas Nauseda confirmed the arrests, thanking Polish authorities.
Three people have been arrested over an attack on a senior aide to the late Russian opposition leader Alexei Navalny.
Leonid Volkov said his arm was broken outside his home in the Lithuanian capital Vilnius in March.
Poland's prime minister said the person who allegedly ordered the attack was "a Belarusian working for the Russians".
Two Polish citizens linked to football ultra-fan groups are accused of carrying it out.
Lithuanian intelligence originally said the attack was likely "Russian-organised".
At a press conference on Friday, Lithuania's President Gitanas Nauseda confirmed the arrests, thanking Polish authorities.
In a tweet on X, Mr Nauseda expressed his gratitude to Polish President Andrzej Duda and warned the organisers of the crime to "not try to repeat it!"
Two Polish citizens were being held by police in Poland's capital Warsaw, after being detained on 3 April, the Lithuanian prosecutor's office said.
They also added that the crime was committed due to Volkov's "beliefs and his political activities".
Polish police said the men were wanted under a European Arrest Warrant and were being held at Warsaw-Praga district prosecutor's office.
Polish Prime Minister Donald Tusk said a Polish man was arrested and charged separately for planning to co-operate with Russian intelligence services to aid an attempted assassination of Ukraine's Volodymyr Zelensky.
"There will be no leniency for collaborators of the Russian services. We will burn down every betrayal and attempt at destabilization," he said in a statement posted on X, formerly known as Twitter.
Mr Volkov also tweeted his reaction to the arrests, saying: "I don't know any other details yet, but I can say: I have seen how energetically and persistently the Lithuanian police have worked over the past month on this case, and I am very glad that this work has been effective."
Back in March, the activist said the incident was "an obvious, typical, gangster greeting from Putin, from bandit St Petersburg", but gave no more details about his attacker.
"Everything happened in silence... of course this is a clear political attack, there is no doubt here," he said.
Kremlin officials declined to comment on the arrests during a news conference on Friday.
Mr Volkov was briefly hospitalised after the attack, which took place as he arrived at his home in Vilnius by car on 12 March.
At the time, pictures were posted on social media by another member of the Navalny team showing Mr Volkov with bruising to his head and a bloodied leg, after he was hit with a hammer.
Mr Volkov, 43, was Navalny's chief-of-staff and has lived outside Russia for some years for his own safety.
Russia's late opposition leader died suddenly in prison in the Russian Arctic in February.
This week, his widow Yulia Navalnaya said Mr Volkov's attack had prompted her to hire a bodyguard.
Speaking to Time magazine, she said: "Some people were waiting for him with a hammer and a can of pepper spray. His wife was in the house with their little kids."
In the article, Ms Navalnaya also said neither she nor her late husband used to have security and that she had inherited his "courage".