Some eight special trains and 400 coaches are ferrying in supporters from across the country for the “Italian Pride” demonstration, with the crowds also set for a speech from former prime minister Silvio Berlusconi.
Salvini, head of the far-right League party, pulled support from the previous populist government over the summer in a bid to spark elections he was convinced he could win to govern the eurozone's third-largest economy alone.
That plan failed when his former coalition partner, the anti-establishment Five Star Movement, sealed a deal with the centre-left Democratic Party to form a new government. But after suffering a blip, the League's popularity has risen again in opposition.
Recent polls put the anti-immigration party at between 30 to 33 percent of voter intentions, well ahead of the Five Star (M5S) and Democratic Party (PD), which have dropped slightly to between 18 and 20 percent each.
With the current left-leaning government seeking to change the electoral law to prevent Salvini triumphing alone at the next elections, the 46-year old hopes to unite parties on the right and centre-right under his leadership.
Forza Italia head Berlusconi, 83, whose party has been in a lengthy slump, appears open to just such an alliance, along with the smaller, far-right Brothers of Italy.
Salvini in August had refuted the idea of a tie-up with Forza Italia, saying the League “needs nothing and no-one”. Nevertheless, Salvini has a reputation for changing his mind so often on so many issues that he should come with a warning that his statements were “irreversibly reversible”, editorialist Mattia Feltri wrote this week in the Corriere della Sera newspaper.
Neo-fascist party CasaPound is also expected at Saturday's rally, while a small counter-protest will be held in a nearby square.
City needs love
Political analysts say Salvini has set his sights on taking Rome and hope the right-wing alliance could carry him to victory in key upcoming regional elections, potentially setting him up for a win on a national level.
He “is doing what he fundamentally does best: opposition on the ground. Among the people,” said the Open news website.
The next general election is not due until 2023, but the current governing coalition of former foes is shaky and may not last.
Salvini has waged war on Rome's mayor, M5S member Virginia Raggi, calling for her resignation, and will circulate a popular petition Saturday demanding she step down now, two years before her term is due to end.
The League head took part in a sit-in against Raggi earlier this month. He then did Facebook live videos from places he says symbolise the city's decline, from an abandoned stadium to a residential area besieged by illegal dump sites.
“We need a mayor capable of loving this city and cleaning it up,” he said to Raggi, telling her to go back to being a mum.
'Hands off Rome'
Raggi, 41, has come under intense fire for the city's ongoing garbage crisis and beleaguered transport services, which have existed for decades. She has blamed the problems on organised crime and corruption in previous administrations.
“Hands off Rome,” she tersely replied to Salvini on Twitter.
The League leader has found an unlikely ally in his battle against Raggi in former prime minister Matteo Renzi.
Beyond that, the two Matteos profess to have little in common. As Salvini rallies Saturday, Renzi will be drumming up support for his new centrist Italia Viva party at a Florence convention.