An anonymous caller threatened to kill former Chief Minister Mulayam Singh Yadav's son Akhilesh Yadav and his personal assistant, police said on Thursday.
The caller ringed Akhilesh Yadav's personal assistant Gajendra Singh on Wednesday night and started abusing him and threatened to kill him and Yadav, a Samajwadi Party MP, they said.
Singh reached the residence of Senior Superintendent of Police (SSP) and informed him about the incident.
An FIR has been registered in the matter in Hazratganj police station, Pawan Pandey, Lucknow University Student Union vice-president who accompanied Yadav, told PTI.
Superintendent of Police, Crime, has been directed to probe the matter. Police is trying to trace the caller through details of his mobile number.