The Delhi Police has registered a case against unidentified people for recording and circulating an audio message containing the threat to remove the Indian flag from Pragati Maidan, the venue for high profile G-20 meet in September.
The case was registered at the complaint of a man who received the pre-recorded message on his phone when he arrived at the Delhi airport.
In the message, the alleged Khalistan supporter was talking about taking over Pragati Maidan and removing the Indian flag. Later, the person also allegedly talked about 'Waris Punjab De' chief Amritpal Singh, they said.
A case under sections 153 (provocation with intent to cause riot), 153A (promoting enmity between different groups on grounds of religion etc) and 505 (statements conducing to public mischief) of the Indian Penal Code was registered at IGI Airport Police Station, they said.
The case has been transferred to the Special Cell unit of the Delhi Police, they added.
Singh, the radical preacher, is on the run after the Punjab Police arrested several of his supporters in a major crackdown that began weeks after the group stormed the Ajnala Police Station near Amritsar to secure the release of an arrested associate.
The episode had raised fears over the possibility of the return of Khalistani militancy to the state that borders Pakistan.
Police said efforts are on to trace Amritpal Singh. A lookout circular and a non-bailable warrant has been issued against the Khalistan sympathiser.