A homeless man with a history of mental illness randomly attacked a 6-year-old boy in front of his grandparent’s house in Queens on Thursday afternoon, seriously injuring the boy, police and sources said.

The child was sitting on the steps of his grandparent’s house on Metropolitan Avenue near 123rd Street in Kew Gardens at about 5 p.m. when the vagrant walked into their driveway, said Rabbi Naftali Portnoy, the boy’s grandfather.

The 35-year-old vagrant then grabbed the child, picked him up and threw him to the concrete, slamming his face on the ground, police said.

His brother rushed into the house and told his grandfather about the attack.

Portnoy called 911 and tailed the vagrant, who was shirtless, while he walked away from the scene down Metropolitan Avenue.

Cops arrived soon after and arrested the suspect, who sat down on the street and said, “I’m bipolar,” the grandfather explained.

The kid was rushed to Cohen Children’s Medical Center and treated for hemorrhaging to the brain and facial contusions, a source said.

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“Please keep the child in your prayers,” Portnoy said after the attack.