Police in Rome said officers seized sharpened iron bars, clubs and knives ahead of anti-EU march - just a few miles from where the EU summit was being held.
Hours before the marches started on Saturday, police checked the occupants of cars, trucks and buses heading to Rome.
Police said one van inspected at a highway toll had knives and gas masks. In Rome, not far from one of the routes of the marches, traffic police found a bag filled with sharpened iron bars.
By early evening the marches were finished and several thousand protesters were dispersing.
One protest banner read: "Politicians, bankers and eurocrats, our patience is over."
Monuments and museums closed down as a precaution, and streets in the centre of Rome streets were largely deserted except for the marchers.
Italian state TV RaiNew24 reported that police stopped dozens of protesters from reaching twin anti-EU marches.
One protest leader, Tommaso Cacciari, complained that 150 protesters, arriving on buses from north-east Italy, were prevented from joining the marchers.
One anti-EU march moved past the iconic Mouth of Truth monument while another march route flanked the Tiber River. The protests grouped opponents to the EU, NATO and the high-speed rail project in northern Italy.
The Colosseum was closed and museums and many shops closed up, fearing clashes.
Earlier, pro-EU marches went off peacefully in Rome.