Dozens of protesters have thrown bricks and firecrackers at a police blockade, with officers using tear gas in response. Austria's plans to tighten border controls have angered many Italians.
Video footage posted to social media Saturday showed a line of riot police being attacked by projectiles and smoke bombs just a few hundred meters from the Italian border with Austria.
Police fired tear gas and water cannon to disperse the protesters, many of whom were hooded or wore helmets or gas masks. Italian newspaper Corriera della Sera reported this week that the protest had been organized by an anarchist group from Trentino, northern Italy.
The Italian news agency Ansa reported that about 500 people had gathered close to the Brenner Pass border to demand a reversal of a raft of Austrian anti-migrant measures, which include new border controls on the Austria-Italy border.
Four police officers and several demonstrators were injured. Some 20 people were arrested, according to the German news agency dpa.
The protest spilled over on to the railway, blocking train services, and then onto the highway. Italy's state television RAI showed police detaining one demonstrator near some railway tracks.
Saturday's protest went ahead despite a prohibition zone being placed around the mountainous Brenner Pass after a similar demonstration on Friday.
Free movement 'at risk'
After receiving 90,000 asylum applications over the past year, Austria has speeded up plans to restrict access through the Alpine crossing and to build a fence to prevent a new influx of refugees arriving through Italy.
The Austrian government said it was forced to take preventive measures after another major migrant route, through Greece and the Balkans, was shut off, leaving the Alpine route vulnerable.
The Europa Bridge, just south of Innsbruck, forms part of the main route across the Alps from Austria to Italy
The Brenner is a key route between Austria and Italy and until recently, there were no border controls as the crossing is part of Europe's borderless Schengen zone.
Since January 1, over 28,500 migrants have landed in Italy, with many of them then heading to other European countries.
EC Commissioner warns of consequences
Also on Saturday, European Commission President Jean-Claude Juncker waded into the debate over the new border controls, saying they would be a "political catastrophe" for Europe.
"Everything that blocks the Brenner Pass will have not just serious economic consequences, but most importantly heavy political consequences," he said, referring to the route being a major European transport corridor.
mm/jm (dpa, AP)