* League chief says won't give in to European budget demands
* Rightist League now the most popular party in Italy
* Salvini warns that austerity leads to street violence
Italian Deputy Prime Minister Matteo Salvini filled one of Rome's main squares with tens of thousands of flag-waving supporters on Saturday in a rally that underscored the growing predominance of his rightist League party.
Locked in battle with Brussels over the government's budget plans, Salvini told the cheering crowds he would honour promises to boost state spending, warning that austerity would only fuel the sort of violence that has rocked neighbouring France.
"Those who sow poverty reap protests," he said.
"If you call into question our right and duty to restore dignity, pride, security, pensions and work to millions of Italians, then I tell you we will not be turning back."
Bearded and stocky, the plain-speaking Salvini took charge of the League in 2013 when it was a small, scandal-plagued regional party scoring around three percent in the polls.
Under his leadership, and fuelled by anti-migrant rhetoric, the League has become a national force that won 17 percent of the vote in a parliamentary election in March, enabling it to form a coalition with the anti-establishment 5-Star Movement.
Since taking office, Salvini has made good on promises to slow drastically the arrival of mainly African migrants into Italy, and has seen support for the League surge to around 34 percent, making it the largest party in Italy.
League voters from around the country flowed into the huge Piazza del Popolo (People's Square) under a warm winter sun, their flags sporting the names of cities from the south, such as Bari and Naples, and to the north, such as Venice and Milan.
"WE ARE ONE COUNTRY"
In its separatist past, the League's rallying cry used to be "Roma Ladrona" (Thieving Rome), but such slogans have now been replaced with calls for national unity. "Italy lift up your head," said a huge banner on the main stage.
"We need to overcome our old divisions. We are one country and Salvini is the only man who can save us," said Ivana Di Betta, 62, a resident of the northern city of Udine, who caught one of 200 especially chartered buses to attend the rally.
Salvini vowed that the governing coalition would survive a full five-year term, though it is proving less popular in Brussels, where the European Commission has rejected its 2019 budget. The EU executive says the budget will not lower Italy's large public debt as the rules require.
Salvini said the European Union should focus on fostering growth rather than be obsessed by "decimal places" and reiterated a pledge to lower retirement ages and cut taxes.
"Someone has betrayed the European dream, but we will put blood and strength back into the veins of a new European community," he said. (Editing by Gareth Jones)