Beppe Grillo wrote on his blog that they are both “two men alone at the helm”.
"There are disturbing similarities between Andreas Lubitz, the co-pilot of the Germanwings Airbus A320 that crashed in the French Alps and Matteo Renzi, who is at work to make Italy crash," he said.
In an image on his blog, Renzi is also depicted as a pilot flying a plane with the Germanwings log on the side and an Italian flag on the front.
Lubitz was named by French investigators as being solely responsible for the Germanwings disaster in the French Alps on Tuesday, after locking the pilot out of the cockpit and forcing the plane to crash.
READ MORE: What we know about the Germanwings co-pilot
The plane had been making its way to Dusseldorf from Barcelona when it crashed in between the towns of Digne-les-Bains and Barcelonnette.
Grillo compared Lubitz locking the pilot out of the cockpit to Renzi “eliminating the Senate and any internal opposition”.
“The Airbus passengers realized only at the last minute that the co-pilot was taking them to disaster, after eight long minutes. Italy will also only understand at the last minute, when there will be nothing left to do,” he added.
A spokesperson for Grillo declined to comment when contacted by The Local.
But the comments were met with an immediate backlash.
“Even the right to freedom of expression must have a limit: not to cross the boundary of bad taste, not to use tragedies,
pain and collective anguish as material for the dig of the day. This is what Grillo has done," Walter Verini, leader on the
House justice committee for Renzi's Democratic Party (PD) was quoted by Ansa as saying.
PD politician Stefano Pedica said “only someone mad can attempt to profit from the death of 150 people using it as an excuse to attack Renzi,” adding that Grillo should drop politics and go back to being a politician.
Even his supporters lambasted the comments.
“Dear Beppe, you’ve really hit the bottom,” one wrote beneath his blog post.
“What a shame! We were very much deluded by the force of your ideas and united with your battles, even in the voting booth.”
“Disrespectful is an understatement,” wrote another.
“Congratulations for the comparison. With a sense of shame and sorrow, I apologize to all the families involved in this family. What a shame.”
Italy's flagship airline Alitalia told The Local on Friday that it has changed its rules to ensure there are two people in the cockpit at all times following the tragedy.