The five Star Movement has hailed the result as a "moment of glory"
An anti-establishment party that was set up less than ten years ago is set to become the biggest political party in Italy.
Exit polls suggest the Five Star Movement will be the big winner in what was a volatile campaign – however the party has pledged not to enter a coalition.
The result means the country is facing the prospect of a hung Parliament – and fraught negotiations to form a Government.
As polls closed at 11pm local time (10pm Irish time) on Sunday, the populist and anti-establishment party looked set to win with 32.5% of the vote in the lower house.
It was also looking likely to have 33% of votes in the upper house Senate, making it the largest single party there by a large margin.
Five-Star deputy Alessandro Di Battista described the results as a "triumph", adding: "This is a real moment of glory.
"Everyone will have to come and talk to us."
The Democratic Party, which currently heads the government under leader Matteo Renzi, was the next largest party.
They had less than 20% of the vote, according to the figures compiled by Piepoli polling agency.
Michele Martina, a minister in the outgoing government, said the result was a "very clear defeat."
Mr Renzi may now resign, according to Italian media.
Former Premier Silvio Berlusconi's Forza Italia and the anti-immigrant League were polling at between 13.3% and 17.7%.
They are running as part of a three-party centre-right coalition and, together, looked set to gain the most votes - 35.5%.
According to Rai TV, the coalition would win between 225 and 265 seats in the lower house, far short of the 316 needed for a majority.
Five-Star would get between 195 and 235 seats.
Earlier, a topless protester leapt on to a table in front of Mr Berlusconi as he was about to vote.
Scrawled across her bare chest were the words: "Berlusconi, your time has run out."
The exit poll has a margin of error of 3%.
The election was held amid a weak economy, widespread distrust of the political class and growing anxiety over migration.
The centre-left was hit by widespread anger over persistent poverty, high unemployment and an influx of more than 600,000 migrants over the past four years.
Euroscepticism has also been rising in Italy, a country where pro-EU sentiment used to be rock-solid.
Election turnout was 71.48%, the Interior Ministry said, a slight drop from the 75% of eligible voters who participated in the 2013 election.
Reporting from IRN ...