As a choir sang, Pope Benedict XVI poured holy water over Allam's head and said a brief prayer in Latin.
"We no longer stand alongside or in opposition to one another," Benedict said in a homily reflecting on the meaning of baptism. "Thus faith is a force for peace and reconciliation in the world: distances between people are overcome, in the Lord we have become close."
Vatican Television zoomed in on Allam, who sat in the front row of the basilica along with six other candidates for baptism. He later received his first Communion.
Allam, 55, told the newspaper Il Giornale in a December interview that his criticism of Palestinian suicide bombing provoked threats on his life in 2003, prompting the Italian government to provide him with a sizable security detail.
The Union of Islamic Communities in Italy-which Allam has frequently criticized as having links to Hamas-said the baptism was his own decision.
"He is an adult, free to make his personal choice," the Apcom news agency quoted the group's spokesman, Issedin El Zir, as saying.
Yahya Pallavicini, vice president of Coreis, the Islamic religious community in Italy, said he respected Allam's choice but said he was "perplexed" by the symbolic and high-profile way in which he chose to convert.
"If Allam truly was compelled by a strong spiritual inspiration, perhaps it would have been better to do it delicately, maybe with a priest from Viterbo where he lives," the ANSA news agency quoted Pallavicini as saying.
The nighttime Easter vigil service at St. Peter's Basilica marked the period between Good Friday, which commemorates Jesus' crucifixion, and Easter Sunday, which marks his resurrection.
Benedict opened by blessing a white candle, which he then carried down the main aisle of the darkened basilica. Slowly, the pews began to light up as his flame was shared with candles carried by the faithful, until the whole basilica twinkled and the main lights came on.
The pope administers baptism "without making any 'difference of people,' that is, considering all equally important before the love of God and welcoming all in the community of the Church," said the Vatican spokesman, the Rev. Federico Lombardi.
Allam, who has a young son with his Catholic wife and two adult children from a previous relationship, indicated in the Il Giornale interview that he would have no problem converting to Christianity. He said he had even received Communion once-when he was 13 or 14 - "even though I knew it was an act of blasphemy, not having been baptized."
He did not speak to the press Saturday and his newspaper said it had no information about his conversion.
Allam said in the interview that he had made a pilgrimage to Mecca, as is required of all Muslims, with his deeply religious mother in 1991, although he was not otherwise observant.
"I was never practicing," he was quoted as saying. "I never prayed five times a day, facing Mecca. I never fasted during Ramadan."
Allam also explained his decision to title a recent book "Viva Israele" by saying he wrote it after he received death threats from Hamas.
"Having been condemned to death, I have reflected a long time on the value of life. And I discovered that behind the origin of the ideology of hatred, violence and death is the discrimination against Israel. Everyone has the right to exist except for the Jewish state and its inhabitants," he said. "Today, Israel is the paradigm of the right to life."
In 2006, Allam was a co-winner, with three other journalists, of the $1 million Dan David prize, named for an Israeli entrepreneur. Allam was cited for "his ceaseless work in fostering understanding and tolerance between cultures."
There is no overarching Muslim law on conversion. But under a widespread interpretation of Islamic legal doctrine, converting from Islam is apostasy and punishable by death-though killings are rare.
Egypt's highest Islamic cleric, the Grand Mufti Ali Gomaa, wrote last year against the killing of apostates, saying there is no worldly retribution for Muslims who abandon their religion and that punishment would come in the afterlife.
On Wednesday, a new audio message from Osama bin Laden accused the pope of playing a "large and lengthy role" in a "new Crusade" against Islam that included the publication of drawings of the Prophet Muhammad that many Muslims found insulting.
Lombardi said Thursday that bin Laden's accusation was baseless. He said Benedict repeatedly criticized the Muhammad cartoons, first published in some European newspapers in 2006 and republished by Danish papers in February.
(© 2008 The Associated Press. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed.)