Thousands of gay rights activists have demonstrated in Rome in support of same-sex unions.
Gay protesters decried the influence of the Catholic Church
A senior judge celebrated "weddings" for 10 same-sex couples, in a ceremony of symbolic rather than legal value.
The move has been denounced by Pope Benedict XVI, who says gay marriage would "obscure the value and function of the legitimate family".
Meanwhile in Milan, in another challenge to the Roman Catholic Church, women marched in defence of abortion.
Police said it had attracted 50,000 people.
Same-sex civil unions - illegal in Italy - have become an important issue ahead of forthcoming general elections, while conservatives and Catholics are behind efforts to tighten the country's liberal abortion law.
The influence of Catholicism was a source of resentment for many of the demonstrators. In Rome, a banner read: "Let's free love from religious phobia."
But members of the right-wing government expressed disregard for the protesters and their cause.
Police said 50,000 people gathered to protest in Milan
"These demonstrators are really nauseating," said Reforms Minister Roberto Calderoli.
The Pope has dismissed gay marriage, saying on Thursday it was "a serious mistake to obfuscate the value and functions of the legitimate family based on marriage by attributing legal recognition to other forms of legal union for which there is no real social demand".
Gay rights activists retorted: "What a pity the Pope has not taken advantage of the occasion to affirm that Christianity means love and acceptance of all people."
Gay marriage and the legalisation of cohabitation arrangements between unwed couples of the same or the opposite sex has become a political hot potato in Italy, says the BBC's David Willey in Rome.
Italy's left-wing opposition has said it will change the law if it wins the April general election to allow common-law partners of opposite sexes to obtain legal recognition of their unions, but it stopped short of proposing to allow gays and lesbians to wed.
The leader of the left, Romano Prodi, who could become Italy's next prime minister if Silvio Berlusconi is defeated in the general election, is caught in an awkward dilemma, our correspondent says.
He and his wife are practising Catholics and are under pressure from the Church to uphold traditional Catholic family teaching and values.
The latest statistics, however, show that common-law unions have doubled in Italy during the last 10years, but they still represent only a small minority of couples - 6% in the north and only 2% in the south of the country.
Gay marriage is already legal in several European countries, including traditionally Catholic Spain.
Last month Britain introduced a law allowing same-sex couples to formalise their relationships.