The CGIL union estimated that some 61% of all public and private sector workers had joined the strike, while more than 500,000 people marched in rallies in Italy's main cities.
The CGIL has some 5 million members, half of whom are pensioners.
Police estimates of the number of demonstrators were not immediately available.
"The majority of the workers who keep this country going don't agree with and don't accept this government's budget," CGIL chief Maurizio Landini told a rally in Florence. "Today shows more clearly than ever that we need a change."
The CGIL, along with other unions, has criticised the budget's planned increases in defence spending and call for more investment in healthcare and education and measures to bolster salaries and pensions.
The government says the budget, which has been well-received by financial markets and ratings agencies, strengthens Italy's public finances while lowering taxes for middle earners.
Italy has seen a spate of national strikes and marches in recent months against the government's economic policies and its support for Israel.
Meloni's right-wing Brothers of Italy party comfortably tops opinion polls and her ruling coalition remains stable more than three years after winning power.
Some recent surveys, however, show a decline in the government's popularity and a modest rise in support for the centre-left opposition. The next election is due in 2027.
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