By Stephen Brown ROME, Feb 14 (Reuters) - The Italian government warned on Wednesday of possible violence during a protest at a U.S. military base this weekend and faced ridicule from the opposition for having to urge its own ministers not to attend. Leftists, greens, pacifists and locals will demonstrate on Saturday against the expansion of the U.S. base at Vicenza which is home to the 173rd Airborne Brigade. The demonstration has become a delicate issue for the ruling majority since many of those who will protest are centre-left voters who helped elect the government last year. Prime Minister Romano Prodi, who has faced revolts by his leftist coalition partners on everything from gay rights to the budget and Italian peacekeepers in Afghanistan, put his foot down saying the government "cannot protest against itself". Two leftist cabinet ministers ceded, but support the march in absentia, while other officials, including four communist and Green under-secretaries, agonised over whether to join in. Interior Minister Giuliano Amato asked politicians of all hues to "show solidarity with the forces of law and order". "This could be an opportunity for all those elements who are hostile to the forces of law and order to unite," he said in a speech to parliament on Wednesday. Officials put the march in the context of the arrest this week of 19 leftists linked to an offshoot of the Red Brigades, said to be planning new attacks. The Brigades kidnapped and killed officials in the 1970s and '80s and as recently as 2002. The U.S. embassy has warned Americans to steer clear of Vicenza and authorities said schools which have classes on Saturday would be shut to ensure students' safety. March organisers criticised Amato for "evoking the spectre of possible incidents" and linking the march to the arrests, saying any violence would be the work of "provocateurs". While leftists wrestled with their consciences -- Economy Undersecretary Paolo Cento of the Greens said the base caused pollution and was a launch pad for "sending soldiers to wage war in Iraq" -- the opposition enjoyed the spectacle. "This confirms that the government no longer has a majority on foreign policy, since on Saturday Prodi will be defied in the streets by his own governing majority," said acid-tongued Senator Roberto Calderoli of the right-wing Northern League. But one leftist minister who opposes the U.S. camp, Social Affairs Minister Paolo Ferrero, said for him it was clear "a minister cannot take part in a protest which has elements of criticism against the government. You cannot have more than one role in a play."
