The crowd is made up of protesters from various anti-government rally sites including Pathumwan, Asok and Lumpini.
The group led by PDRC core members Suthep Thaugsuban, Putthipong Punnakant, Sathit Wongnongtoey and Anchalee Paireerak planned to submit a letter to Pol Gen Adul Saengsingkaew.
The national police chief will be asked to appoint neutral police officers, as opposed to "pro red-shirts", to investigate the deaths and injuries of PDRC protesters and ensure that justice is served.
Speaking through loudspeakers amid noisy whistle-blowing, the PDRC core members called on the police to swiftly solve the cases and identify the culprits for punishment.
Pol Gen Adul reportedly appeared at the front gate at 10.13am. He opened the gate to allow some protesters inside. The police were not armed and did not obstruct protesters.