The father of a man who let his baby pee on the metro, and at least one other passenger, has apologized publicly for his son's behavior, local media reported Sunday.

The grandfather went to the metro's public security bureau and wrote an apology letter to the other passengers Saturday, according to a report on the news website xinmin.cn. In the letter, he said his son was too young "to mind his behavior."

The baby's father was riding Line 3 around 6:50 pm Thursday with his father, wife and son when the little boy felt nature's call, the report said.

The mother cradled the baby in her arms and let him pee on the floor of the train car, according to a six-minute video of the incident that a microblog user posted online. A woman standing nearby complained that some of the urine splashed onto her shoes.

Other passengers joined the woman in berating the family. The father, who was sitting next to his wife, began shouting and threatening the passengers who criticized his family, the report said.

He stood up from the bench and tried to kick the woman who complained about her shoes, but the grandfather restrained him.

At one point, the father called a friend to come help him beat up one of the male passengers who insulted his son. In the video, a male passenger compared the baby to a dog. The grandfather also threatened the passenger.

The incident set off a storm of criticism online against the family for their uncivilized behavior, though some Internet users pointed out that the other passengers were also rude.