Participants in the city of Verona carried pink and blue balloons and placards with slogans such as “Yes to life, not to abortion”, an AFP correspondent saw.
Supporters travelled by train and coach from all over Italy to attend the march, which came a day after protesters staged their own demonstration denouncing the organisation's anti-abortion, anti-gay stance.
Before the march started at around midday, those gathered listened to a message from the organisers of the Congress.
“The family, fundamental pillar of our society, must be at the centre of government policies,” they said in their closing statement.
Some marchers carried tiny rubber models that were handed out during the conference, which were intended to represent a 10-week old foetus. They came with the message: “Abortion stops a beating heart.”
The three-day conference attracted senior political figures from the far-right including Italy's Interior Minister Matteo Salvini and two other government ministers.
Don't touch abortion law: Salvini
But the event also revealed divisions in the ruling coalition of Salvini's League party and the anti-Establishment Five Star Movement. But Salvini's coalition partner Luigi Di Maio, leader of the Five Star Movement, denounced the World Congress of Families as a gathering of “fanatics”.
“The vision defended by this Congress in Verona is a vision of the world that belongs for the most part to the Middle Ages, which considers women as submissive,” said De Maio.
Salvini, while backing the Congress's vision of a family as one composed of “a father and a mother”, rejected calls by some participants to abolish Italy's laws.
“You don't touch the 194 law,” said Salvini, referring to the 1978 legislation that legalised abortion. “There's no debate either on abortion, or on marriage. Everyone makes love with whoever they want, and dines with whoever they want,” he added.
Italy legalised civil unions between people of the same sex in 2016. Salvini has however spoken out against gay couples being allowed to adopt. Founded in 1997 by the American Brian Brown, the World Congress of Families has held an annual meeting since 2012.
Its credo, as stated on its website, is to “affirm, celebrate, and defend the natural family as the only fundamental and sustainable unit of society”.
Previous meetings include one in Hungary, which enjoyed the patronage of the country's far-right Prime Minister Viktor Orban.
As well as Salvini, other speakers at this year's event include the President of Moldova Igor Dodon — a close ally of Russian President Vladimir Putin.
Hungary's ultra-conservative Families Minister Katalin Novak and a senior figure in the Russian Orthodox Church, Dmitri Smirnov, were also listed as speakers.
And two other Italian ministers were listed as speakers at the Congress: Lorenzo Fontana, Minister for Families and Disabilities and Education Minister Marco Busetti.