Lobbing rocks and swinging pieces of wood, thousands of protesters clashed with police who used tear gas and clubs to keep them from reaching a forum on information technology in Naples.
Although the protest was called to denounce globalization, some turned out to publicize other causes, Italian news agencies reported.
About 70 demonstrators and 50 officers were slightly hurt, police said.
The Group of 8--Russia, Japan and six Western industrial powers--will hold its annual summit in the Italian city of Genoa in July. President Bush is expected to attend.