Police estimated that between 15,000 and 20,000 people attended a campaign rally in Milan led by Matteo Salvini, the head of anti-immigration group The League, which said the turnout was closer to 50,000.
The group, formerly known as the Northern League, is part of ex-prime minister Silvio Berlusconi's right-wing coalition, which opinion polls suggest is leading ahead of the March 4th election.
"Italians first," Salvini, who hopes to become the country's next leader, told the crowd.
Violence briefly broke out at another Milan demonstration, police in riot gear briefly grappling with anti-fascist protesters marching to counter a rally by the neo-fascist CasaPound movement.
There has been a dramatic uptick in violence between anti-fascist and far-right activists in recent weeks, and more than 3,000 police were mobilised for demonstrations in Rome.
However, the biggest march in the Italian capital remained calm, with a sea of red and green union flags waving in the cold rain.
"We are here to say no to fascism, because there are too many toxic incidents at the moment, it must not come back," Rita Solo of the CGIL union told AFP.
Prime Minister Paolo Gentiloni and former leftist PM Matteo Renzi, who is vying to regain the post, briefly attended the demonstration, which was organised by the National Association of Italian Partisans (ANPI), under the slogan "Fascism Never Again".
Protesters hurled firecrackers at police at another protest in Rome, which was organised by a left-wing union to rail against labour reforms.
Earlier in the day in Milan, hundreds of anti-fascist activists, many of them holding Communist Party flags, had gathered in a square to protest.
"We find it really shocking that two demonstrations by racist parties were authorised in Milan," said local activist Gianni Fossati.
The climate was tense in Palermo, Sicily ahead of an evening rally by the extreme-right Forza Nuova group, after one of the party's activists was beaten up by men wearing masks there on Tuesday.
There have been numerous clashes between anti-fascist and far-right activists across the country, particularly after a racially motivated attack on February 3rd by a far-right gunman in the central city of Macerata that left six African migrants wounded.
The violence comes amid fears of a revival of far-right groups in the election, after several European countries shifted to the right in recent years against a backdrop of economic and migration woes.
An average of the last major opinion polls suggested Berlusconi's right-wing coalition, which includes his Forza Italia (Go Italy) party, The League and the far-right Brothers of Italy could finish on top, but fail to achieve a parliamentary majority.
"Salvini represents change for Italy. It's a key moment," said Diego Bazzano, a 30-year-old member of The League who turned up to the Milan rally.
"We Italians no longer have any rights, no work. We can no longer take in people who come in from abroad and then commit crimes," he said.
"We are no longer free to live as we want."