Officers intervened after concerned members of the public spotted the trapped pooch in central London. It comes as the UK is experiencing record-breaking temperatures with parts of England and Wales under a red warning for extreme heat.
It also comes amid warnings from animal charities not to leave pets in hot cars as the mercury soars.
Eyewitness John Thompson said: “I was just immediately scared for the dog and thought come on, it’s the hottest day of the year.
“The dog was panting on the passenger seat and then moved down to where the pedals are because it was cooler at the bottom.
“Two people from the theatre next to the car had spotted it as well and somebody else in a van said they had been watching it for about 10 minutes.
“One of the people from the theatre called 999 and the call handler said if it was still there and panting in the next five to ten minutes then they could smash the window.
“They went back into the theatre and came back with a hammer and by that point, it had been fifteen minutes – someone said it takes fifteen minutes for a dog to succumb to heatstroke.
“They were just about to smash the glass when the police turned up – they checked the air con which wasn’t on and then tried to smash it with a baton and the hammer which just bounced off but after using a little glass breaker it smashed immediately.
“The owner turned up. They argued back and forth for 10 minutes or so until the police took the dog and put it in the back of their car with the air-con on.
“The woman was really upset because she thought they were taking the dog but the police returned the dog and they left in their car with one window smashed.”
A Met Police spokesperson said: “At 6.16pm, on Monday, 18 July, police were alerted by a concerned member of the public to a dog inside a car with the windows closed in Oxendon Street, SW1.
“The dog, which had allegedly been in the car for some time, appeared to be suffering in the heat.
“Officers attended and broke a window to gain entry to the dog and the owners of the car were spoken to and their details were taken.
“The dog appeared in good health once it had been released from the car.”
Meanwhile, a man was arrested in a separate incident in Hertfordshire on Tuesday when a dog needed emergency veterinary care after being left in a car.
Herts Police tweeted: “We’ve arrested a man in South Mimms after a dog was found in a car with all the windows and doors shut.
“The RSCPA is being contacted & the dog is having emergency treatment. This follows a similar incident in Stevenage. Don’t leave pets in cars in this weather.”
And last Thursday, police smashed the window of a vehicle in Kew in southwest London after a dog with a muzzle on was left for over an hour.
Kew Police said on Twitter: “Unbelievable end to the shift. Leave your dogs in a hot car, less than an inch of gap, no water, muzzled, for over an hour and you better know we are going to be smashing that window.
“Absolutely heartbreaking. Thankfully dogs are now fine. Owner to be interviewed under caution.”
It comes as the RSPCA has warned owners not to leave their dogs in vehicles and is urging people to call 999 in an emergency if they see a pet suffering in a hot car.