Tanvir Javed told police that his 23-year-old son Tahir Malik was going to Defence area on a motorbike Wednesday evening when he had a heated exchange with Abdul Qadir, Gilani's eldest son, and his security guards, The Nation reported.
"Gilani junior's guards on a double cabin signalled Tahir to get aside to clear the way for the VIP. As Tahir took a little time to move to the extreme left of the road, a guard opened fire on him, killing him on the spot," Javed said.
Following the incident, Javed along with his family placed his son's body outside Yousuf Raza Gilani's residence and demanded registration of a murder case against Qadir and his guards.
Imran Khan's Pakistan Tahreek-i-Insaf workers also joined the protesters.
Imran Khan has declared a "war on VIP culture" in Pakistan.
A heavy contingent of police reached the spot to stop the protesters from entering Gilani's house.
After several hours of protest, the police booked Qadir and his five guards on murder and terrorism charges.
"We have arrested one of Qadir's guards, Muhammad Khan, after registering a case against him and his five guards," a senior police official said.
He said the statements of eyewitnesses have been recorded.
"We have included terrorism charges in the FIR as killing someone on the road without any provocation amounts to creating terror in society," the official said.
Qadir told the police that since his family had been under severe security threats after the kidnapping of his younger brother, Ali Haider Gilani in May 2013, his guards opened fire on Tahir, mistaking him for an "assailant".
"We offer our deep regret to the family of Tahir," Qadir said.
The case was apparently registered after Punjab Chief Minister Shahbaz Sharif took notice of the incident and ordered the Lahore police chief to probe the matter.