A small homemade bomb injured six people at an election rally in Bologna where the leader of Italy's National Alliance (AN) was addressing a crowd ahead of European polls this weekend.
Television pictures showed injured people with blood on their faces being taken into ambulances, and what appeared to be bits of metal on the pavement after the blast in Piazza Maggiore, Bologna's main square.
A police spokesman said the effects could have been worse if the incendiary device had succeeded in causing a fire. "It was a rudimentary device made from a bottle containing fertiliser and a detonator," he told reporters.
Two of those injured were taken to hospital while the other four were treated by medics on the scene.
The AN, lead by Mr Gianfranco Fini, is the second-biggest party in Prime Minister Silvio Berlusconi's centre-right coalition government. A small bomb damaged one of its offices in Sardinia earlier this week.
Mr Berlusconi said his coalition would not be deterred by the attack: "I don't think a bomb is going to take away our desire to carry on the electoral campaign," he told reporters as he arrived in the United States for a meeting of world powers.
Ansa news agency said initial suspicions of investigators were focused on anarchist groups. The theory may gain credence from the fact the latest blast was in the traditionally leftist city of Bologna, where a group blamed by authorities for a series of letter bombs in December and January is thought to be based.
European Commission President Romano Prodi, who has a home in Bologna, was among those targeted by the letter bombs.