Stones were thrown at CPI leader Kanhaiya Kumar's cavalcade in Bihar's Saran district on Saturday afternoon, police said.
The incident took place when the cavalcade was passing through the Chapra-Siwan main road at Kopa bazaar, they said.
Sources said 20-25 youths attacked the cavalcade with stones.
Kanhaiya Kumar was not injured in the incident. The window panes of one vehicle of Kumar's cavalcade were damaged, SHO, Kopa, Shivnath Ram, said.
Those behind the attack are yet to be identified, he said, adding that the miscreants managed to escape from the spot.
Kumar was going to Chapra from Siwan to participate in his "Jan Gan Man Yatra", police said.
The former JNU students leader's car was redirected to the Kopa police station by his supporters to save him from the miscreants, sources said
On January 31, Kumar had to face stiff opposition from supporters of Bharatiya Janata Yuva Morcha in Gopalganj where his posters were blackened and slogans of "go back" raised.
Later addressing a public meeting at Marwari high school ground in Muzaffarpur on Saturday evening, Kumar said, "We will defeat this (NDA) government and save the Constitution.