The car belonged to the officer in the Nusaybin district of Mardin province.
He was about to leave for the rally in Siirt province when his friend, also a police officer, noticed an object under the vehicle.
The bomb disposal squad was called, which carried out a controlled explosion to destroy the device, while a further investigation is underway.
President Recep Tayyip Erdogan attended various ceremonies in Siirt during the day.