Neither Five Star nor the centre-right (LN/FI) coalition can govern alone, but it seems likely that the future administration will feature one anti-establishment party or another.
"We have the right and duty to govern", League leader Matteo Salvini told a news conference.
Indeed, the emergence of both The League and the 5-Star Movement - which have both rallied against European Union bureaucracy - will have shocked Brussels to its core.
The rightist alliance that also includes former prime minister Silvio Berlusconi's Forza Italia (Go Italy!) was on course for about 37% of the vote - but for the first time the League emerged as the senior partner.
Despite overseeing a modest economic recovery, the ruling center-left coalition trailed a distant third on 22 percent, hit by widespread anger over persistent poverty, high unemployment and an influx of more than 600,000 migrants over the past four years.
Election turnout was 71.48%, the Interior Ministry said, a slight drop from the 75% of eligible voters who participated in the 2013 election.
What does the result mean?
"We expect lengthy negotiations after these elections, which may lead to increased volatility of Italian assets", said Matteo Ramenghi, CIO at UBS WM Italy.
The maverick populist Five Star Movement (M5S) was the night's success story, sweeping Italy's south to become the single leading party in terms of votes. According to preliminary results, the PD obtained only 19 % of the general vote, with the possibility of winning 117 seats in the lower House and 59 seats in the upper House, behind the M5E, with 229 and 114 seats; and the conservative coalition, with 267 and 135, respectively.
The results will not be final until Monday evening, but exit polls point to a remarkable shift in Italian politics away from conventional parties to anti-establishment groups looking to shake up business as usual. Five Star performed better than anticipated and was forecast to take the most seats - around 102-122 - but miss out on a majority.
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Five-Star would get between 195 and 235 seats.
The other would be a minority Five Star government, which could prove highly unstable.
Migration proved to be a key issue for Italian voters.
Germany, on the other hand, struck a blow for normalcy, as coalition talks finally concluded and Merkel pledged to work with the "Social Democrats" for the good of Germany. She tweeted: "The spectacular advance and top showing of the League [Lega] coalition led by our ally and friend Matteo Salvini is a new stage in the awakening of the people!"
So, which way will Five Star seek to turn? A potential alliance between the two would be viewed with some degree of worry in Brussels.
A Northern League/5-Star coalition would be a boon for the Kremlin which has had to weather several election disappointments in Europe after its successes in 2016 when voters in the United Kingdom opted to leave the European Union and American voters handed the White House to Donald Trump - both seen as major foreign policy wins for Putin.
He has promised to deport hundreds of thousands of migrants and spoken of the "danger" of Islam.
"The League has won within the center-right, and it will now lead the center-right", Salvini stressed.
In any case Mr Berlusconi, 81, can not hold public office himself until next year because of a tax fraud conviction.
Throughout the campaign, Berlusconi was careful to cast himself in the role of the "elder statesman", even tapping in the last days of the campaign European Parliament President Antonio Tajani to take on the role of front-man-prime-minister.