At least 50,000 people attended a demonstration for Europe promoted by journalist Michele Serra in Piazza del Popolo in Rome, organizers said.
The square, which was surrounded by barriers, was overcrowded and thousands of demonstrators gathered around it to listen to the speakers.
Many others walked up to the Pincio Terrace above the square to follow the event from a large monitor.
A large number of demonstrators were waving the flags of the European Union and peace.
The pro-Europe rally was attended by many representatives of centre-left parties, lawmakers, labour union members, associations and mayors, inculding Rome's Roberto Gualtieri.
It included activists and politicians who are both in favour and against European Commission President Ursula von der Leyen's ReArm plan.
Opening the event, Serra said "there are many of us because we are a population".
"A European square is a square of people who think differently on a number of things", the journalist also said.
"Each one of you could have beside them a person who votes for another party, who does not believe in God or in your God.
"Democracy is not very fashionable in the world.
"In a world that appears to be in pieces, a square that gathers different people and ideas is a scandal.
"This scandal has a name - it's called democracy", said Serra.
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