Premier Silvio Berlusconi suffered a heavy blow after two days of local polling across Italy, losing his stronghold Milan and the southern city of Naples, projections and partial returns showed on Monday.
The defeat further weakens Mr Berlusconi, already hit by sex scandals and falling approval ratings, and will likely deepen rifts with government ally, the Northern League, which had been critical of the electoral campaign in Milan and will be furious to have lost the north's most important city.
Milan had been run by conservative mayors for almost two decades.
With about half of polling stations reporting, Mr Berlusconi's candidate, incumbent Mayor Letizia Moratti, had garnered about 45% of the vote in the run-off against Giuliano Pisapia of the centre-left.
The figure was confirmed by projections released by state-run RAI shortly after the polls closed on Monday afternoon.
In the Naples run-off, the leftist candidate Luigi de Magistris, a former magistrate, was leading with 65% of the vote compared to 35% for Mr Berlusconi's candidate Gianni Lettieri, according to partial returns.
The centre-left has long controlled Naples but Mr Berlusconi had been hoping to grab it as the southern city still grapples with a long-standing rubbish collection crisis and high unemployment rate, especially among the youth.
The run-offs were called for candidates who had failed to garner the 50%-plus-1% of the vote in the first round of voting two weeks ago.