Mr Kejriwal was atop an open jeep when a man wearing a red t-shirt jumped onto the vehicle and slapped the chief minister before being pulled off the jeep.
The 33-year-old man, identified as Suresh, who is a scrap dealer in the area, has been a supporter of the Aam Aadmi Party and used to work as an organiser of the party's rallies and meetings, they said.
"An enquiry by a DCP-level officer has been ordered to enquire as to how this person was allowed to be in the reception or proximate group," Anil Mittal, Additional PRO (Delhi Police) said.
According to his version, over a period of time, Suresh got disenchanted due to behaviour of the AAP leaders. His anger intensified due to "distrust of the party in the armed forces", the official said, adding further interrogation is on in the matter.
No FIR has been registered in the matter as police did not receive any complaint.
"Today, he was wearing a cap (which he later took off) and scarf of the AAP, and was in the reception group of the CM. No one objected to him being there as he had been an organiser for the party. He was standing near the front right tyre of the Gypsy. He took off the scarf, climbed the bonnet and attempted to assault the CM," Delhi Police said in its statement.
The AAP, however, alleged that the Delhi Police had planted that the man belonged to the party.
The AAP roadshow was organised from 4 pm to 10 pm in Moti Nagar. It was scheduled to start from Karampura and was to terminate at RK Ashram Marg, it said.
Proper police arrangement from both Security Unit and local police was put in place for the event in consultation with the organizers of the event, it added.
The chief minister arrived at around 5.43 pm at the starting point. He got out of the official vehicle and boarded the open gypsy prepared for the roadshow. As he was meeting and greeting his party workers who had gathered around the gypsy, suddenly a person got on to the bonnet of the vehicle and attempted to assault the chief minister, the statement said.
He was immediately overpowered and saved from the agitated supporters and taken to hospital for medical treatment. The roadshow then started and continued as per the schedule, it said.
During security arrangements at such events, which are put in place in consultation with the organizers, necessary tie-up is made with the organizers so that they ensure that only the persons identified by them are in the reception party or the proximate group or near the vehicle used for the roadshow, police said.
AAP spokesperson Saurabh Bharadwaj alleged that the Delhi Police had planted the man.
"Delhi Police planting that man belonged to AAP, this is really shameful given the fact that the attacker's wife has herself said he was a Modi Bhakt and did not like anyone talking against Modi.
This is the same Delhi Police which had planted earlier that no ''mirchi attack'' happened on the chief minister. It was later when the Delhi government provided CCTV footage to Delhi Police that left its political masters red faced," Mr Bharadwaj said.