"A bearded man with an air gun was detained. His air gun was seized along with his bag," a district police official said.
Police said the man was spotted carrying an air gun and trying to enter the site of the Gandhi's rally ahead of state polls.
Officials interrogated him but refused to share details with media.
The rally will be the first big show of the grand alliance after the 'Swabhiman rally' in Patna on August 30, which was attended by Congress president Sonia Gandhi, RJD chief Lalu Prasad and Bihar Chief Minister Nitish Kumar.
Nitish Kumar and Lalu Prasad will not be present and due to their engagement with finalisation of candidates.
However, JD-U general secretary and national spokesperson KC Tyagi and other senior party leaders will attend the rally.