The rally's slogan was "Renzi go home", with Salvini, "the other Matteo", setting himself up as a challenger to Italian Prime Minister Matteo Renzi.
"I do not make the distinction between the right and left, the struggle between fascists and communists belongs to the history books. We differentiate between those who work and parasites, and defend those who work," Salvini told crowds at the rally.
Ten thousand people had gathered, organisers said. However, the number was not confirmed by the police and the Piazza del Popolo did not seem completely full.
According to Italian media reports 3,000 police were deployed to deal with the rally where organizers bused in supporters on special trains and buses.
A few hundred metres from the Northern League rally, leftist groups, including anti-racism and gay rights organisations, demonstrated under the banner "Never with Salvini" with organizers saying 35,000 had gathered to protest.
Salvini, 41, is hoping to make gains in the regional elections set for early 2015 in Tuscany, Marche and Latium, the region that includes Italy's capital.
Salvini a self-confessed fan of France's far-right leader Marine Le Pen, has a strong anti-euro stance and advocates a more rigorous application of immigration laws.
"Our main objective can be summed up in three points: no to the euro, retrieve monetary sovereignty, stop immigration. There is no more room for anyone else with the unemployment rates we have today. And finally, Italians first," he said.
Salvini's party, which is also close to the Italian far-right group CasaPound, has evoked strong reactions from opponents.
A few hundred metres (yards) from the Northern League rally, leftist groups, including anti-racism and gay rights organisations, demonstrated under the banner "Never with Salvini" with organizers saying 35,000 had gathered to protest.
Despite the opposition, Salvini's party's popularity has been growing steadily in the polls, even overtaking Silvio Berlusconi's Forza Italia.
Salvini however has refused to discuss possible arrangements for May's regional elections with Berlusconi's party.