PHOENIX (Reuters) - A marijuana-smoking woman was arrested on Saturday in Phoenix after she accidentally drove away with her five-week-old son in a child safety seat on the roof of her vehicle, police said.

The baby fell off the car in the middle of an intersection and was found unharmed and strapped into the seat, said Phoenix police spokesman James Holmes.

The mother Catalina Clouser, 19, was booked into jail on child abuse and aggravated assault charges, he said. The infant was taken to a local hospital as a precaution and is in the custody of state Child Protective Services.

"It appears the suspect put the baby on the roof of the car and drove off, forgetting he was still on the roof," Holmes said in a written statement.

Police said Clouser and her boyfriend had been smoking marijuana in a park and left with the toddler to buy beer late on Friday night. Officers stopped the car and the boyfriend was arrested for suspicion of driving under the influence while driving with the baby in the 2000 Ford Focus.

Police learned that Clouser was so upset about the arrest that she drove to a friend's home and "admittedly smoked one or two additional bowls of marijuana," Holmes said.

She left at about midnight with the baby asleep in the car seat, placing the child on top of the vehicle, he said. Clouser apparently did not realize that the baby was missing until she arrived home.

Holmes said witnesses who were friends of the mother advised officers that the child belonged to Clouser. The mother then arrived at the scene and "made admissions to what had occurred."