"I have called everywhere, police stations, hospitals, Facebook and I can't find my son. I have been looking for him for 48 hours," he told AFP.
"My wife is dead, where is my son?"
Mejri is one of hundreds whose life changed in an instant when a truck careered into Bastille Day crowds in Nice on Thursday.
A few hours later his search came to an end at the Pasteur Hospital in the north of the city where he learned that his son was dead.
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It was, but 31-year-old Tunisian Mohamed Lahouaiej-Bouhlel smashed through onto the pavement in the truck, leaving police helpless to stop him from killing 84 and injuring around 200.
"Everyone was there, old people, babies," Mejri raged.
Abandoned dolls and pushchairs were among the debris left along the promenade after the driver was shot dead by police.
Ten children and teenagers were among the dead and another five children were still fighting for their lives alongside dozens of critically injured adults.
"There were a lot of head injuries and fractures," said hospital spokeswoman Stephanie Simpson.
Two of the children admitted to the hospital died shortly after the attack.
Simpson said the youngest victim being treated was six months old.
Also in the hospital was an eight-year-old boy who had yet to be identified.
Romanian authorities said three of their citizens were missing, and one of them might be the boy at Lenval.
"We are used to receiving a lot of children at the same time, but this, has been hard to manage. It is the psychological aspect," said Simpson.
One man accompanied his 13-year-old daughter and ex-wife who had been to see the fireworks display and got caught up in the chaos after the attack.