Yelena Vasilyeva was returning home after the party's founding conference in the central Russian town of Tambov on Tuesday night, when a man came up to her saying "We will teach you not to travel to Tambov" and hit her in the chest, the radio said.
The woman lost consciousness and was taken to hospital in a nearby town. Vasilyeva said the attacker had shadowed her in Tambov, the opposition Kasparov.ru website quoted Vasilyeva as saying.
This is the second attack on a member of the opposition party in two weeks.
Lev Ponomaryov, leader of the For Human Rights movement and a Solidarity member, was attacked outside his east Moscow home last Tuesday.
Ponomaryov's relatives and colleagues linked the attack to his political activity.
The Paris-based Reporters Without Borders condemned the assault, urging the authorities to intervene and take measures to prevent frequent attacks on human rights activists and journalists.
Sergei Protazanov, a reporter for the Grazhdanskoye Soglasie newspaper, based in the north Moscow suburb of Khimki, died in hospital last Monday, two days after he was beaten near his home.
