Salvini was sworn in as interior minister on Friday after striking a last-gasp deal to resurrect a coalition with Five Star Movement head Luigi Di Maio, an agreement that has now brought Italy a populist government after almost three months of post-electoral deadlock.
Initially the demonstration was called to demand the impeachment of Matarella, when the President refused to accept eurosceptic Paolo Savona as Minister for Economy.
ROME, May 30 (Reuters) - Italy's two main anti-establishment parties could yet form a government, after the man nominated as interim prime minister said politicians, rather than technocrats like himself, might be able to steer the country out of deadlock.
The Italian president reappointed law professor Giuseppe Conte as Prime Minister-designate on Thursday in order to lead a coalition government.
The 53-year-old academic heads a government of ministers from the anti-establishment Five Star Movement (M5S) and the far-right League, the first populist coalition in a founding European Union member.
Prime Minister May congratulated Prime Minister Conte on his new role and on forming a new government.
In an address given on Friday, President Mattarella expressed his "best wishes to the new government for its work". The new government features 18 ministers, including five women.
An anti-establishment government took power in Italy on Friday after a last-ditch coalition deal was hammered out to end months of political turmoil, narrowly avoiding snap elections in the eurozone's third largest economy.
The world will be carefully watching power relations between Conte and his two influential deputies - particularly the European Union, given the eurosceptic leanings of Five Star and the League.
A coalition government has been agreed in Italy, ending months of uncertainty in the EU's fourth-biggest economy.
In a recent video on his Facebook page, which has more than two million fans, Salvini said he would work to "stop the landings" once in power. The two parties have a solid majority in both houses of parliament.
Offering the new government cautious support was Italy's small, far-right neo-fascist CasaPound party, which held its own Republic Day commemoration on Saturday.
The initial failure of Conte to form a government had alarmed financial markets, which feared a quick return to the polls that risked being tantamount to a plebiscite on Italy's keeping the euro currency.
"The free ride is over", League leader Matteo Salvini, Italy's new interior minister, warned migrants at a rally in northern Italy.
Mr Conte who has been criticised as being a "Mr Nobody", named hardline anti-migrant Mr Salvini as Interior Minister, while Mr Di Maio will become Minister for Economic Development.
"That must not happen again in the present case with Italy", Juncker said.
Salvini, who in 2009 was caught on video singing songs about "stinking" Neapolitans and in 2012 said the south didn't deserve the euro, represents impoverished southern region Calabria in the Senate and has redirected his regional chauvinism to take the League nationwide.