Cell phone video obtained by Action News shows the first moments the fire started around 11:30 a.m. Friday.
You can see Medic Unit 44 parked inside the fire station at 4th and Arch Streets, flames coming from the engine.
People in the area, many of them tourists, couldn't believe they were seeing a fire inside a fire station.
"We were right down the block," said Ted Gutkowski from Langhorne, Pa. "And I looked down at the firehouse, and I saw black smoke coming out of it. That's not something you normally see."
The firefighters who work there were away at training at the time. The two paramedics assigned to Medic 44 called 911 and tried to put out the flames themselves using a fire extinguisher.
"They did what they could do with the fire extinguishers," said Gutkowski, "but unfortunately they couldn't put it out."
Firefighters arrived on the scene and ran toward the burning fire station with their hoses.
Just then, there was an explosion. Onlookers ducked for cover.
"I came out here really quick," said Janet Stann of Frederick, Maryland. "You could see the whole building was engulfed in black smoke. And then when the smoke cleared, you could see that the ambulance was the cause of the explosion."
The medic unit was destroyed, the building damaged by smoke and flames.
This fire station will be out of service while the cause is under investigation and repairs made.
"We're trying to find out what we're going to do as far as placing our members in other stations," said Fire Commissioner Lloyd Ayers. "So we have a pretty arduous task in front of us for the rest of the day."
Fortunately, no one was injured.