Police arrived after the ambulance crew reported their vehicle had been driven away from outside an address in Pevensey Road, Eastbourne, East Sussex, as they were preparing to take the baby to hospital on Sunday.
A police spokesman said yesterday: "They reported at 12.40am that a taxi driver had objected to the road being temporarily obstructed, so he had just got into their ambulance and drove it a short distance up the road, before driving away in his own taxi.
"The ambulance crew retrieved their vehicle and took the baby to hospital."
The 64-year-old driver went to the police station of his own accord a short while later and was arrested on suspicion of taking a vehicle without consent, and obstructing an emergency worker. He was bailed pending further inquiries.
A South East Coast Ambulance Service spokeswoman said it could be frustrating when an ambulance had to block a road, but this was only done to ensure staff got to those who needed their help quickly.