Omar Manneh, 42, had been walking to a shop in Chapeltown shortly before 5pm when he spotted his sister covered in blood.
His eyes then fell upon his “happy, charming” nephew, Yusupha Jatta, who was lying on the ground in Chapeltown Road.
The boy – the youngest of four siblings – had been struck by a car as he crossed the road, just a short walk away from his family’s home in Reginald Mount.
Mr Manneh said: “I didn’t witness the accident but I was there immediately afterwards. I was going to neighbourhood shop when I saw my sister covered in blood.
“I saw my little nephew lying down. The ambulance arrived and they were trying to revive him.
“Can you imagine a mother seeing her little boy knocked down? I saw the panic in my sister. She can’t do anything about it.”
Video: Tributes to boy, four, knocked down by car in LeedsAppeal: Police in Leeds seek witnesses after death of four-year-old boy hit by carYusupha was taken to Leeds General Infirmary, where he was pronounced dead.
Mr Manneh, who lives in nearby Harehills Avenue, said his sister and her husband were too upset to speak about their loss.
Yusupha’s 11-year-old sister, who was with her mother and brother at the time of the collision, is also traumatised by what she witnessed.
Last night she repeatedly asked her uncle who she would play with now that Yusupha was gone.
“When I listened to her talking, I thought life can just take things,” Mr Manneh said.
“Whenever I saw Yusupha and his mum, he was always active. He was a charming boy.
“He was an intelligent little lad – happy, charming, truly family.”