ROME - Thousands of protesters took to the streets across Italy Monday to protest the austerity measures adopted by Prime Minister Mario Monti's emergency cabinet.

In Rome, demonstrators gathered outside the parliament after major trade union leaders called on workers to oppose the measures, which they considered too tough for retirees and workers but not harsh enough for the rich.

"We recognize the measures must be rigorous, but we also want them to be fair," head of the centrist CISL labor union, Raffaele Bonanni, was quoted as saying by state TV channel Rai 3.

Rome authorities have also intercepted two letters containing bullets addressed to the capital's mayor and the Italian justice minister. A local anarchist group claimed responsibility for the letters.

Protests also broke out in other cities. In Florence, thousands of people marched on the streets, shouting "more fairness." In the northern city of Turin, workers of Fiat automaker staged an eight-hour strike, and the demonstration in Milan has forced the iconic La Scala opera house to cancel a concert.

Susanna Camusso, head of Italy's largest trade union CGIL, said the nationwide strike was caused by the government's failure to show willingness to discuss changes in its "irresponsible" plans when meeting with labor unions' representatives Sunday.

Welfare Minister Elsa Fornero defended the measures, saying some pension reforms would be softened, but massive spending cuts were necessary for the country to regain credibility on financial markets, ANSA news agency reported.

The austerity package, to be presented in parliament this week and approved before Christmas holidays, is aimed at raising 30 billion euros (around $40 billion) from public expenditure cuts and new taxes.

It includes a major reform of the pension system with a rise of the retirement age, a property tax, a 2-percent increase in value-added tax and new efforts to fight tax evasion.

The annual tax evasion in Italy is estimated at 255 to 275 billion euros, accounting for 16.3 to 17.5 percent of the GDP, President of national statistics institute ISTAT, Enrico Giovannini, was quoted by local media as saying on Monday.

In recent months, the yields on Italian state bond have soared to levels considered unsustainable by many analysts. On Monday, stock index in Milan closed down 3.79 percent.

To restore market confidence, Italy on Monday raised 7 billion euros in a bond auction, backed by the national banking association to encourage citizens to purchase state bonds.