People have reported seeing football fans stampeding from the square.
Several people have been reported injured due to crushing as thousands of fans tried to flee the area.
“We were all overwhelmed, there are children who lost their parents and relatives”
A witness told La Stampa newspaper: “We heard a bang and everyone started to run.
“We heard screaming and at one point thousands of people began to press to run off the square.
“We were all overwhelmed, there are children who lost their parents and relatives.
“You do not understand anything even after several minutes, it looked like a real bomb.”
Early reports vary on the source of the “explosion” with some suggesting the noise was a banger that spooked fans.