Doctors said they had ordered Berlusconi, 73, to cancel or reduce public appearances at least for the next two weeks, meaning he will skip climate talks in Copenhagen.
They had previously said he would be released Wednesday. Berlusconi's personal doctor said, however, that he was not worried that the prime minister's overall health was in danger from the injuries he suffered when a souvenir metal statuette of Milan's cathedral was hurled into his face at close range at a rally in the city by a man with a history of psychological problems.
Police tightened security around Berlusconi after the attack but saidof the Milan hospital where the premier was being treated early Wednesday morning.
A Milan anti-terrorism police official asserted that the incident did not reflect a new security failure because they had put checkpoints only on the floor where Berlusconi was being treated, and the man had been stopped at the first checkpoint.
The official said the unarmed 26-year-old man from Turin "wanted to pay a visit on the premier at 2 a.m." and said he was a supporter of Berlusconi's.
The official spoke on condition of anonymity because he didn't have advance authorization to talk with a reporter. Police issued no official statement on the incident.
Police said they found hockey sticks and kitchen knives but they believed he plays hockey. He will be cited for transporting the knives without justification and released, the official said.
The man's identity was not immediately released, the official said.
Alberto Zangrillo, Berlusconi's personal doctor, said he expected the premier would be discharged early Thursday afternoon.
"We are not concerned," he said.
Berlusconi's spokesman, Paolo Bonaiuti, said the premier had a "rough night" Tuesday and felt more pain than before. An old neck problem was aggravating the situation, he said.
Bonaiuti said Obama called Tuesday night to offer get-well wishes.
Berlusconi is fending off a sex scandal and legal troubles and he has decried what he calls a "climate of hatred" against him. Berlusconi's injuries have stirred public sympathy but groups praising assailant Massimo Tartaglia mushroomed on Facebook after the attack.
Some of his conservative allies have been urging the government to crack down on Web sites that contain material inciting people to political violence or hatred. Interior Minister Roberto Maroni said a Cabinet meeting planned for Thursday would take up a measure allowing magistrates to remove Web pages that instigate or advocate violence.
Facebook in Italy said it was examining "with extreme attention all the requests of intervention about content" by Italian officials regarding Berlusconi. "We will reply swiftly, removing every kind of content that directly threatens or supports violent acts against persons."
It added it was in contact with Italian authorities about the matter.
A Milan judge on Wednesday denied a request by Tartaglia's lawyers for their client to be transferred from San Vittore prison to the psychiatric ward of a hospital, Italian news reports said. The judge also upheld the arrest warrant, the ANSA news agency said.
Phones went unanswered at the offices of Tartaglia's lawyers.
A half-exploded parcel bomb was found at Milan's Bocconi university overnight, authorities said. It did not cause any injuries. Maroni, the interior minister, said an anarchist group had claimed responsibility for the device and he said the explosion, like Berlusconi's attack, could be attributed to a "climate of exasperated confrontation."