Allies of Italian Prime Minister Silvio Berlusconi have suffered setbacks in local elections seen by analysts as a test of his government's popularity.
His centre-right PDL bloc came second in his stronghold Milan, forcing it into a run-off vote for the first time in nearly 20 years.
The bloc and its Northern League partner also did poorly elsewhere - though the PDL led in Naples.
The second round of voting is due to be held in two weeks.
Mr Berlusconi has seen his reputation tarnished by criminal trials and sex scandals while his government is criticised for low economic growth, unemployment and its handling of the Libya crisis.
The prime minister was said by aides to be "surprised and saddened" by the result in Milan, one of Italy's wealthiest cities.
He said previously that to lose there was "unthinkable".
In the event, the centre-left's Giuliano Pisapia won 48%, beating incumbent mayor Letizia Moratti by 6.4 percentage points.
"Surprise in Milan, Pisapia in the lead," an editorial in the Milanese daily Il Corriere della Sera read.
"The run-off in Milan is humiliating for Silvio Berlusconi, who turned the result into a referendum on both him and his government and has received a slap in the face both personally and politically," said columnist Massimo Franco.
However, he added that the PDL could still recover to win the second round.
"This is not a knockout blow for Berlusconi but it is a blow," political analyst James Waltson of Rome's American University told the Associated Press news agency.
According to the agency, the Northern League lost support in 14 out of 15 major contests.