A defiant Silvio Berlusconi has vowed to stay in politics to reform the very justice system that convicted him of tax fraud, and has threatened to bring down the government of the prime minster who replaced him.

On Friday a Milan court sentenced the 76-year-old to four years in jail - which was quickly reduced to one year under an amnesty law designed to tackle overcrowding in prisons - and banned him from holding public office for five years for tax fraud.

In a shock decision, the court ruled he had inflated the prices paid for television rights via offshore companies and skimming off money to create illegal slush funds.

The sentence came a week after Berlusconi denied in a separate case that he hosted raunchy parties and paid for sex with then 17-year-old exotic dancer Karima El-Mahroug, better known as Ruby the Heart Stealer.

The sex trial was one of the last in a series of scandals that helped precipitate Berlusconi's downfall in November 2011.

The three-time premier had announced he would not run in the next election due in the spring but did not say he was withdrawing completely from political life.

On Saturday Berlusconi told a news conference his centre-right bloc may withdraw its support from the government of Mario Monti, a move that could throw Italy into political chaos ahead of next April's national elections.

He also railed against the Italian justice system and labelled Friday's verdict "intolerable".

"Ours is not a democracy but a dictatorship of the magistrature... I feel obliged to stay in the (political) field to reform the planet justice," the media tycoon said.

"There are going to be consequences."

Asked about the legitimacy of someone who had been convicted to lead a crusade to reform the justice system, he replied: "It is not only just, it is a duty for someone who enjoys the high esteem of millions of Italians."

"[To ensure] what is happening to me does not happen to the citizens of Italy."

Political player

The No Monti Day demonstration protested against Mario Monti's austerity measures ( AFP: Alberto Pizzoli )

Tens of thousands of people marched through Rome in a "No Monti Day" on Saturday, some throwing eggs and spraying graffiti to protest against austerity measures introduced by Mr Monti's government.

In an effort to capitalise on discontent with Rome, Berlusconi condemned the government for following the "hegemonistic" economic policies of Germany.

He also accused German chancellor Angela Merkel and former French president Nicolas Sarkozy of "trying to assassinate my international political credibility" when he was prime minister.

"We have to recognise the fact that the initiative of this government is a continuation of a spiral of recession for our economy," he said.

"Together with my collaborators we will decide in the next few days whether it is better to immediately withdraw our confidence in this government or keep it, given the elections that are scheduled."

The Monti government of non-elected technocrats is supported by the centre-left, the centre-right and the centre.

It would lose its majority and have to resign if the entire centre-right, including Berlusconi's PDL party, withdrew support.

Mr Monti took office as prime minister last November when Italy's bond yields were soaring.

He has pushed through tax hikes, spending cuts and a pension overhaul to cut public debt which is running at 126 per cent of gross domestic product.

Unemployment in Italy has risen to 10.7 per cent, its highest level since monthly records began in 2004, and unions are locked in disputes with companies over plant closures and layoffs.

AFP/Reuters