The founder of Italy's Five Star Movement (M5S) has been widely criticised for defending his son's sexual assault allegations.
Beppe Grillo published a passionate video on his popular blog and lashed out at the media for their reporting on the case.
Grillo's 20-year-old son, Ciro, has been accused of involvement in a group sexual assault in Sardinia in the summer of 2019.
Prosecutors are expected to decide shortly whether to pursue charges against the four suspects in the case.
In the video, Grillo claimed any sexual activity involving his son was consensual, stating that no one had been arrested.
But the comedian has been widely derided for his reaction, which some Italian politicians have labeled "sexist".
Italy's former Prime Minister Giuseppe Conte, who has been tapped as the party's next leader, said he understands Grillo's "anguish as a father" but stressed the need to protect others in the case.
"We cannot overlook that there are also other people ... who must be protected and whose feelings must be absolutely respected, namely the young girl directly involved," Conte said in a statement.
Debora Serracchiani, spokeswoman for the Democratic Party MPs, also condemned Grillo for using his influence to "cast the shadow of blame on a girl who denounced rape".
The family of the 19-year-old alleged victim has also protested against the video.
The widespread uproar over the video could potentially further damage the M5S, which has sunk in opinion surveys.
The party founded by Grillo in 2009 received the most votes in Italy's latest general election before former PM Conte was pressured to resign earlier this year amid criticism over his plans for handling EU recovery funds.