Lev Shlosberg, leader of the regional branch of the opposition party Yabloko, earlier this week attended the burial of two soldiers near the northwestern town of Pskov and was investigating the involvement of local paratroopers in fighting in Ukraine.
An aide to the lawmaker, citing information from relatives, said that some 100 soldiers based in Pskov had been killed in Ukraine.
Russia denies claims it has deployed regular troops to Ukraine.
Yesterday evening the lawmaker was attacked by three unidentified assailants and was hospitalised with head and eye injuries and a concussion, his aide, Alexander Zakharov, told AFP.
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The deputy's wife said Shlosberg appeared to be suffering from short-term amnesia.
"He constantly asked me: 'Where am I?' Zhanna Shlosberg said on Echo of Moscow radio, adding that her husband did not apparently remember that he had been beaten up.
The Yabloko party linked the attack on the 51-year-old lawmaker to his investigation.
"I believe that attack on Lev Shlosberg is connected to his investigation of the deployment of Pskov paratroopers to Ukraine," Yabloko leader Grigory Yavlinsky said on Twitter.
"Who needs this silence? Who can it save?" he wrote in the local newspaper Pskovskaya Gubernia.
"So it has come to us -- a genuine fratricidal war.
"How many people with Ukrainian roots are among Russian servicemen? How many people with Russian roots are among Ukrainian servicemen?"
Kiev and the West said this week that regular Russian troops are on the ground in Ukraine fighting alongside ragtag formations of pro-Russian separatists who have staged a lightning counter-offensive that has turned the tide in the nearly five-month conflict.