The protesters were marching through the southern city in opposition to the Northern League’s anti-EU policies.
People in the immediate area sought refuge in nearby buildings.
Mr Salvini decided to go ahead with his party’s rally in Naples on Saturday despite the planned protests. It is the first time that he has planned to address crowds in southern Italy.
On Friday, a small number of protesters displaying a banner that read “Naples doesn’t want you” occupied the convention centre in Naples where Mr Salvini was due to speak.
The Italian interior ministry had to intervene to ensure that Saturday’s rally could go ahead.
The Northern League has recently tried to broaden its base and make inroads in the south by capitalising on what it believes to be populist, anti-immigrant sentiments in the region.