The Spaniard's car slammed into the track wall at 200 mph after colliding with Esteban Gutierrez's machine before barrel rolling twice.
The wreckage landed upside down on the gravel, but Alonso emerged unscathed.
"I am lucky to be here and thankful to be here. It was a scary moment and a scary crash," said the 34-year-old.
"When I stopped, I saw a little space to get out of the car and I went out quickly just to make sure that my mum, who was watching the race on TV at home, could see that I was OK."
Writing on Twitter, he said he was "aware that today I spent some of the luck remaining in life." He also posted an Instagram photograph of him reading a newspaper headlined: "Luckiest man alive."
Alonso said of the incident: "You are just flying and then you see the sky, the ground, the sky, the ground.
"I didn't know where I was because I was so far from the track. You want to stop, and it doesn't stop. It keeps going and going and going."
Alonso said of the incident: "You are just flying and then you see the sky, the ground, the sky, the ground.
"I didn't know where I was because I was so far from the track. You want to stop, and it doesn't stop. It keeps going and going and going."
