With over 99% of ballots counted, the League won 34.3%, compared to just 6% in the 2014 EU elections and 17% in the Italian general election last year.
The results confirm the reversal of fortunes of the League and its coalition partner, the anti-establishment Five Star Movement (M5S), which celebrated taking 32.5% at the general election but took home just 17% Sunday, May 26.
The League victory and M5S collapse in popularity "is explosive in terms of the consequences for government stability," political analyst Stefano Folli wrote in the Repubblica daily.
"We're not talking tomorrow, or the day after, but the fuse which will lead to the government's collapse has been lit," he said.
The only way for the eurozone's third largest economy to stave off a political crisis would be if the M5S "transforms into the docile servant of the League head which has just cannibalized it," he said.
An editorialist at the Corriere della Sera, Massimo Franco, agreed the EU vote left Italy with "a more fragile government."
The League snapped up votes from both the M5S and the opposition thanks to a hardline stance on migration and savvy multimedia team which bombards Italians with selfies of Salvini in action across the country.
The 46-year old, who celebrated by tweeting a photo of himself grinning and holding a sign saying "top party in Italy," is now likely to try to force the M5S's hand on every plan it has contested since the coalition formed in June 2018.
"I ask for an acceleration on the government program," Salvini said, brandishing Roman Catholic rosary beads.
The main questions at stake are a high-speed rail line between the cities of Turin and Lyon in France and a flat tax proposal.
M5S head Luigi Di Maio bitterly suggested the League had got one over on it because of a relentless mud-slinging campaign to which the M5S had refused initially to retaliate.
"Perhaps we were too silent, too pure at the beginning, and if that was our mistake I take responsibility," he said Monday.
The results place Prime Minister Giuseppe Conte in an increasingly difficult position. The leader agreed upon by deputy prime ministers Salvini and Di Maio is purportedly independent but was a M5S pick.
Analysts say Salvini may be tempted to break up the coalition and join forces with others on the right.
Italy's small far-right Brothers of Italy (FdI) party took home 6.4% of the vote, while billionaire Silvio Berlusconi's center-right Forza Italia party, a historic ally of the League, pocketed 8.8%.
The center-left Democratic Party (PD), which won just 18% at the general election, took home 22.7% of the vote.
The Green party, which recorded significant gains in many other European countries, took just 2.29%.
Salvini's victory had been widely expected, despite the M5S taking advantage of embarrassing corruption scandals involving the far-right party.
The interior minister sparked an outcry in the run-up to the vote after holding aloft a rosary seen by many as a gratuitous prop at a rally in Milan, and calling for the Virgin Mary to carry him to victory.
"Salvini was convinced he could do it. The (corruption) investigations made the League lose 5-6 points, but then he pulled out his rosary. And perhaps he really did get a miracle," Marco Valbruzzi from the Istituto Cattaneo research institute said. – Rappler.com