Massimo Tartaglia, a 42-year-old with a history of psychological problems, wrote a letter to Berlusconi through his lawyers, saying he was sorry for his act, the lawyers said in a communique.
Berlusconi, 73, was struck in the face Sunday as he signed autographs in Milan. Tartaglia hurled a statuette of Milan's Duomo cathedral at the premier, breaking Berlusconi's nose and two teeth and cutting his lips. He was being questioned in jail Tuesday.
His defense lawyer, Daniela Insalaco, told reporters outside Milan's San Vittore prison that she was awaiting a ruling on whether Tartaglia should be sent to a psychiatric unit. It was not clear when that ruling would be issued.
Berlusconi, in a message from his hospital bed, thanked all those who had sent him good wishes. He urged his supporters to remain calm.
"Love always wins over envy and hatred," he said in the message posted on his party's Web site.
Interior Minister Roberto Maroni gave details about Berlusconi's attacker in an address to parliament. He said Tartaglia was carrying pepper spray and a crucifix, suggesting the attack was planned.
Medical officials said Berlusconi must remain hospitalized until at least Wednesday and probably should cancel all public activities throughout the Christmas season.
The medical bulletin issued Tuesday by the San Raffaele hospital in Milan said Berlusconi was still in pain but his condition was not worrying.
"He greatly wants to get over this difficult moment. I would be happy if we could slow him down a little," Berlusconi's spokesman Paolo Bonaiuti told TV broadcaster SkyTg24.
The attack has already forced the cancellation of some of Berlusconi's plans, including a meeting with Turkish Prime Minister Recep Tayyip Erdogan in Rome on Wednesday. Berlusconi's participation at a U.N. climate summit in Copenhagen also appeared unlikely.
Berlusconi's doctor Alberto Zangrillo told reporters the premier is doing better, but is "saddened."
The attack, which shocked Italians, came amid an increasingly tense political atmosphere. For months, Berlusconi has denounced what he calls a "climate of hatred" surrounding him as he fends off a sex scandal and legal troubles.
Berlusconi's injuries have stirred public sympathy - and spite. Groups praising Berlusconi's assailant mushroomed on Facebook in the aftermath of the attack.
Get-well wishes from Italians and foreign officials have poured in after the attack, including from U.S. Secretary of State Hillary Rodham Clinton and French President Nicolas Sarkozy.
"This brutal and pointless attack has shocked us all. I wish him a swift recovery and a rapid return to the leadership of Italy," EU president-to-be Herman Van Rompuy said Tuesday.