Leader of the Five Star Movement and comedian Grillo gestures while speaking during an election campaign for European Parliament elections in Rome Thomson Reuters ROME (Reuters) - Beppe Grillo, the leader of Italy's second largest political party, said on Monday a court had sentenced him to a year in jail for slandering a science professor and compared himself to late South African leader Nelson Mandela.

People sentenced to less than three years in Italy very rarely actually go to jail, but the shaggy-haired founder of the anti-establishment 5-Star Movement invoked South Africa's first black president who spent 27 years in apartheid prisons.

"If ... Mandela ended up in prison, I can go there too for a cause I think is just and that has been supported by the overwhelming majority of Italians ... Let's be brave!" he wrote on his blog.

"Maybe people are scared that 5-Star is getting close to government?" said Grillo, who has been retreating from the front line of his party over the past year as a younger, more moderate generation takes the fore.

Grillo was sentenced and ordered to pay 50,000 euros ($56,525) for slandering science professor Franco Battaglia at a political rally in 2011.

He said at the time he would not pay the state television licence fee and encouraged his supporters to follow suit following a talk show appearance by Battaglia.

The blog showed a clip of the show in question, in which Battaglia describes "disinformation" about radiation at Chernobyl, which in 1986 was the scene of the world's worst nuclear accident.

"You cannot allow ... Battaglia, a consultant to multinationals, to go on television and casually say no one died at Chernobyl. I'll kick you in the backside and throw you off television, I'll report you and send you to jail," Grillo said at the rally.

(Reporting by Isla Binnie; editing by Ralph Boulton)