The 73-year-old premier collapsed after he was apparently struck in the face by a man who was clutching a small statue of the Duomo, the city’s world-famous cathedral.
A shaken Mr Berlusconi was quickly dragged to his waiting car by his security guards and driven to hospital.
Doctors said he had not suffered concussion and had insisted "I’m fine, I’m fine, don’t worry about me" as he was stretchered in from his car. But they said he had bled profusely from his mouth and two of his teeth were loose, one of them fractured.
Paolo Klun, communications director at San Raffaele hospital, said: “The prime minister is suffering from shock but he is quickly regaining his composure.
”He has suffered trauma to the front of his face, around the lips and nose, and he has two loose teeth, one of which is fractured and there may be damage to the nasal bone.
“As a precaution, doctors have said that he should remain in hospital for the night for observation.”
Police arrested Massimo Tartaglia, 42, a local man who has received treatment for mental health issues for the last ten years.
The incident happened at the end of a fiery speech Mr Berlusconi had given to supporters of his People of Freedom party in the square in front of the Duomo.
It is the second time that Mr Berlusconi – who likes to work the crowds and will often go on walkabouts – has been attacked in public. In 2004 he was hit over the head with a camera tripod while strolling in Rome.
The attack happened as the premier was signing autographs in a scrum of people. It immediate raised questions about the wisdom of allowing the leader of one of the world’s leading economies to be so accessible to the public when the weapon could have been a gun or knife.
Only last month it was reported that security had been tightened around him following warnings by intelligence agencies that he was being targeted by Left-wing terrorists. He had also received anonymous letters in which the authors threatened to harm him unless he resigns.
The atmosphere was already febrile before Mr Berlusconi left the meeting. At the beginning of the rally Berlusconi was heckled and whistled by at least 10 members of the audience who chanted "shame" at him.
The Italian defence minister, Ignazio La Russa, who witnessed the attack, said: "The prime minister was punched in the face by a man who was holding something in his hand. I was about a metre away and I saw blood coming from the premier’s nose and mouth. and he was then pushed into the car by his bodyguards and driven away.
"He was actually trying to step on the edge of the door so that he could stand and wave to the crowds but his bodyguards would not let him do it they pushed him inside.
"The attacker was held immediately by the police and it’s a good job he was because he risked being lynched by the crowd there would have been just pieces of him left."
Umberto Bossi, leader of a coaltion partner the Northern League, said: "This was an act of terrorism. We have had a heavy climate for some time now and this is the first worrying sign. After what has happened today we need to raise our guard."
Mr Berlusconi’s larger-than-life premiership and private life have attracted fierce criticism as well as the adoration of many Italians. He was condemned for imposing a law that granted him immunity from prosecution and last week he had to hit back at claims from Mafia supergrasses that he had been connected to organised crime.
Hundreds of supporters had gathered for Sunday evening’s rally and there was a heavy police presence as Mr Berlusconi arrived with his party faithful chanting "Silvio, Silvio".
Opponents were on one side of the square and there was heckling between them and Berlusconi during his speech in which he again hit out at suggestions he was in in the Mafia.
He told the rally: "I have been described as monster but I am a good son."
When he was heckled he hit back and told them they should be ashamed for "not allowing free speech."
Mr Berlusconi then added that claims he was in the Mafia were "lies" and that he had had enough of "politically motivated magistrates" who he claims are out to get him.