Leaders of Navalny's organisation knew they were facing "high individual risks", Leonid Volkov said in an interview filmed on Tuesday hours before an unidentified attacker assaulted him outside his home.

"They know that Putin not only kills people inside Russia, he also kills people outside of Russia", he said. "We live in very dark times".

Lithuania has accused Moscow of being behind the attack. Volkov blamed Russian President Vladimir Putin.

"This is an obvious, typical criminal 'hello' from Putin, from criminal Petersburg", Volkov wrote on Telegram. "We will keep on working and we will not surrender."

There was no immediate comment from Moscow on the incident.

In his interview with Reuters hours before the attack, Volkov said the death of Navalny last month in an arctic prison had inspired Navalny's supporters to ensure that his sacrifice was not in vain.

"Alexei's death was a devastating loss, and it's a bleeding wound (in) our hearts, but also of course it produced a lot of energy, a lot of political momentum," he said. "It's our task and responsibility to convert this energy into meaningful political action that would make Putin weaker."

Navalny's followers believe he was killed in prison by the Russian authorities. Moscow says he died of natural causes.

Volkov said he drew encouragement from initial moves by Navalny's widow Yulia Navalnaya's to assume her husband's mantle, and from the tens of thousands who had risked arrest by turning out in Moscow for his funeral. The number of small private donors to Navalny's Anti-Corruption Foundation, where Volkov is chief of staff, had risen by half, he said.

Nevertheless, he said, the death of Navalny was a severe blow.

"The situation is terrible... It can't be underestimated - there is no other Navalny, no spare Navalny, no other person like him," Volkov said.

Navalny "had quite a unique ability to frame ideas in a way that were able to ignite millions. We don't have any ability to compensate for this loss."

Volkov said the West was partly to blame for failing to impose consequences on Putin for his actions. U.S. President Joe Biden had said after meeting Putin in Geneva in June 2021 that there would be "devastating" consequences for Russia if Navalny were to die in prison, Volkov said.

"Do you see any devastating consequences? No. Is the West actually able to impose any devastating consequences on Putin? I doubt it."

The task now facing the opposition was to stay united and keep working.

"It's our job to not let it die, because it's a very important part of his legacy. Not to let it fall apart due to internal conflicts, or something like this", said Volkov.

"These are very dark times for us, very challenging ones. But of course we have a very strong determination to carry on."

(Reporting by Andrius Sytas in Vilnius Editing by Peter Graff)

By Andrius Sytas