"This latest manifestation of small-scale politicking did not surprise us," Alexander Udaltsov said in a statement.
"Mr. Šimašius had long had an itch regarding the name of the park outside the Russian embassy and when a section of the street in front of the Russian embassy in Washington was named after Nemtsov, he could not help but ape," he said.
The Vilnius City Council on Wednesday voted to name the park in Vilnius' Žvėrynas neighborhood after Nemtsov.
Vilnius Mayor Remigijus Simasius said that Nemtsov has become "a symbol of democratic movement, resistance".
Russian opposition politician and journalist Vladimir Kara-Murza, who was present at the council's meeting, said that the Lithuanian capital's move is important, because Moscow refuses to allow even a small commemorative mark on the bridge where Nemtsov was gunned down on February 27, 2015.