Trains, planes, schools and even opera houses were hit, as were hospitals which operated with only skeletal staff to deal with emergencies.
Italy's top three unions urged their 11 million members to join some 100 demonstrations across the country.
Reports said that some 200,000 demonstrators marched through Italy's business capital Milan to voice their opposition to the planned reforms.
"There are more than 200,000 of us," Giogio Roilo, an official of Italy's biggest union, the CGIL, told a rally in central Milan.
Like France and Germany, Italy is trying to reform its pensions system which swallows about 15% of gross domestic product.
The government wants to prevent Italians retiring before they have made 40 years of contributions or reached a minimum age of 65 for men and 60 for women, whereas at the moment one can retire at 57 if one has paid into the system for 35 years.
But a poll published in left-leaning daily La Repubblica said six out of 10 Italians were against raising the retirement age.