By Silvia Ognibene

FLORENCE, Italy (Reuters) - Matteo Salvini, the leader of Italy's anti-immigrant Northern League, said on Saturday he would run for prime minister at the next election as the country nears a referendum that could unseat the government.

Prime Minister Matteo Renzi has staked the future of his centre-left government on a constitutional reform that will be put to a referendum vote on Dec. 4.

Salvini, 43, led a rally against the reform on Saturday in Florence, traditionally a centre-left city where Renzi was mayor before becoming prime minister.

Speaking to an estimated 10,000 people in the central Santa Croce square, Salvini announced his candidacy by hailing Donald Trump's election as U.S. president and Britain's vote to leave the European Union.

"If Brexit and Trump's victory teach us something it is that it's not the time to be afraid," he said.

"Today I put myself on the line. I am not afraid of anything or anyone ... We set off today ... to go there and win," he added, advocating measures to protect domestic goods and against illegal immigrants.

Concerns that Italy could become the next nation to be hit by a global populist revolt drove its borrowing costs to their highest for over a year on Friday.

Italy's centre-right has been in disarray since former Prime Minister Silvio Berlusconi was expelled from the Senate and banned from office following a conviction for tax fraud in 2013.

Polls show that if Italy's right-leaning parties were to unite under a single leader they would have some chance of competing with Renzi's PD party and the anti-establishment Five Star Movement, which both stand at about 30 percent of voting support each.

The right-wing Northern League and Berlusconi's centre-right Forza Italia party trail behind on about 12 percent each.

Salvini called for Renzi to resign if voters rejected the constitutional reform, which would reduce the role of the Senate and take back power from the regions.

At the start of his campaign Renzi repeatedly pledged to quit if he lost, but in recent months he has refused to confirm this. Italy's next general election is scheduled for 2018.

(Writing by Valentina Za; Editing by Helen Popper)