"The Five Star Movement has become the only opposition, the only possible change, " said Mr Grillo.
Mr Grillo's Five Star Movement had 25.54% and the centrist list led by Mario Monti 10.57%.
The stand-up comedian who founded the Five Star Movement is the maverick in this campaign.
He predicts there will be another election within a year "at most", and the Five Star Movement will win.
BBC: Beppe Grillo: Italy's rabble-rouser shaking up politics
Pier Luigi Bersani was rebuffed by the anti-establishment Five Star Movement on Wednesday.
The citizens' protest network, the Five Star Movement, saw in all this the sort of old-style politics that it detests.
And this is why the Five Star Movement could have success here, maybe as much as 20-25% of the vote.
The last published polls suggest that the Five Star Movement is probably in third place, ahead of Mr Monti's coalition.
But this election will always be remembered for the stunning performance of the new citizens' protest network - the Five Star Movement.
Beppe Grillo, the leader of the Five Star Movement, is too distractive.
Its latest poll ratings were little better than those of the maverick Five Star Movement led by Beppe Grillo, a blogger and comedian.
It is reflected in the rise in Italy of the anti-establishment Five Star Movement, which was the success story in the recent Italian elections.
Italy's elections are turning into a four-way fight, with the anti-establishment Five Star Movement led by Beppe Grillo, a comedian-activist, polling around 15 per cent.
But how will the Five Star Movement wield its newfound influence?
But, he says, the only people who feel threatened by the Five Star Movement are politicians who have failed, not citizens who are demanding better government.
BBC: Beppe Grillo: Italy's rabble-rouser shaking up politics
In a surge in support, Beppe Grillo's anti-austerity Five Star Movement attracted 25.54% of the vote, making it the most popular single party in the lower chamber.
The third strongest force to emerge from the Italian election, the Five Star movement led by former comedian, Beppe Grillo, has refused to take part in a coalition.
In a climate of widespread public disillusionment with politics, comedian and blogger Beppe Grillo is also making gains by capturing the protest vote with his Five Star Movement.
Several polls now suggest that the most popular option among voters after the centre-left Democratic Party is the Five Star Movement, led by a comedian and blogger, Beppe Grillo.
In a surge in support, Beppe Grillo's anti-austerity Five Star Movement attracted more than a quarter of the vote, making it the most popular single party in the lower chamber.
One possible reason may be fear within both major political parties that comedian Grillo and his anti-establishment Five Star Movement could take an even bigger portion of the electoral pie.
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The Five Star Movement, riding a wave of popular protest and discontent, is expected do well in the polls, but its leader Beppe Grillo has ruled out making alliances with other parties.
The third strongest political force to come out of February's election, former comedian Beppe Grillo's Five Star movement, could not be persuaded to join a coalition and is expected to be in opposition.
The third strongest force to emerge from the Italian election, the Five Star movement led by former comedian, Beppe Grillo, has refused to take part in a coalition with the two main parties.
Just over half of Italians that voted cast their ballots in favor of either the center right alliance of former Prime Minister Silvio Berlusconi or the Five Star Movement of former comedian Beppe Grillo.
Known as the "clown prince" of Italian politics, Beppe Grillo is a 64-year-old comedian turned politician whose Five Star Movement (M5S) is winning the hearts and minds of Italians fed up with mainstream parties.
The centre-left leader proposes reforms to tackle corruption and create jobs, possibly with the backing of comedian Beppe Grillo, whose Five Star Movement won an unexpected 25% in the election and holds the balance of power.
The Italian election, in contrast, is taking place after a two-week opinion poll blackout, amid mounting investor anxiety that former Prime Minister Silvio Berlusconi and maverick outsider Beppe Grillo, leader of the Five Star Movement, may make a strong showing.
Despite the return of former Prime Minister Silvio Berlusconi, blamed by many for having put Italy on its current track, and the rise of comedian Beppe Grillo, whose anti-establishment Five Star Movement has rejected political alliances and carries the momentum, Italian bonds were yielding 4.65% on Monday, only 315 basis points above German bunds.
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"Italy has taken a step forward, " said Davide Barillari, a candidate for Mr. Grillo's Five-Star movement.
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