Andriy Illienko, a Svoboda Party MP, and party activist and lawyer Sydir Kizin were leaving a police station in central Kiev when they were set upon by a gang of youths.
Around a dozen men reportedly kicked and punched the two men but fled when the attack was interrupted by two passers-by, who Svoboda said had rescued their members from "probable death".
An ambulance crew at the scene told The Independent on Sunday that the MP had suffered a fractured jaw and concussion.
Shortly before the attack, Mr Illienko had been speaking to officers at Shevchenko District police station about an incident in which the Svoboda Party accused police of conspiring with nationalist marchers who threw flaming torches at a Kiev hotel on New Year's Day.
In a statement responding to the assault on Mr Illienko, the party said:
"The Svoboda Party considers this attack as a particularly cynical attempt on the life of a statesman. This attack was planned in advance, with thugs and police acting as a coalition."
A recent string of attacks has targeted the opposition and pro-EU activists who have led five weeks of protests against the government's rejection of a free trade and political integration agreement with the EU in favour of closer ties with Russia.
On Christmas Eve, a pro-EU activist, Dmitry Pylypets, was stabbed outside his apartment in Kharkiv, eastern Ukraine.
Opposition journalist Tetyana Chornovol was beaten unconscious after assailants drove her car off the road on Christmas Day.
In the past month, police have raided an opposition headquarters and three critical news outlets, seizing hard drives and computer servers.
Authorities have twice used force to try to clear protesters from Kiev's Independence Square.
Police announced yesterday that they had opened an investigation into the latest attack but had no suspects.
Source: The Independent