The man, identified as Kuldeep Yadav, was caught "red-handed" by some BJP workers yesterday and handed over to the police.
Yadav has been detained for questioning at Parliament Street police station, a senior police official said.
There were allegations that the man took money from several BJP workers, claiming to be the PA of a senior party leader.
"A complaint has been received and we are looking into it. The person is being questioned," DCP (New Delhi) Jatin Narwal said.