Tens of thousands of anti-war protesters gathered in Rome on Saturday to protest against US President George W. Bush, who was in the Italian capital on an official visit.
Organisers said 150,000 people took part in 'No Bush-No War' march but police put the number at 12,000.
The protest kicked off at 16:30 and wound its way from Piazza Repubblica near the city's main train station to the famous square of Piazza Navona.
The march was organised by some 200 pacifist organisations ranging from trade unions and student groups to activists angry at the expansion of a US military base in the northern city of Vicenza.
It went peacefully until the tail end got close to Piazza Navona, when a group of some 100 demonstrators clad in black and wearing hoods and helmets began throwing bottles, stones, smoke bombs and other objects at police in riot gear.
Three policemen were slightly injured while other demonstrators trying to stop the violence clashed with the militants, who also smashed the window of a bank.
Police subsequently tried to disperse the violent protesters by charging them with batons. At least one youth was reported injured.
Security officials expressed concern ahead of the march that militants known as the 'black block' could move down to Rome after taking part in protests in Germany against the Group of Eight summit which has just ended there.
The US embassy warned Americans who happened to be in Rome to avoid the rally, saying that they could turn violent and that they could find themselves targeted.
Organisers of the protests insisted the events would be peaceful and accused security officials of trying to discredit the demos by creating unnecessary alarm.
Meanwhile, in another well-known Rome square, Piazza del Popolo, several parties within Premier Romano Prodi's own centre-left governing alliance - the Communist Refoundation Party (PRC), the Italian Communists' Party and the Greens - staged a separate protest against Bush's foreign policies.
The protest drew little support and was branded a "flop" by the centre-right opposition.
The participation of MPs from Prodi's nine-party coalition - which ranges from Communists to centrist Catholics - was a potential source of embarrassment for the government, which appealed to ministers to stay away.
Several leftist and pacifist MPs took part in the main march as well, which chanted slogans and waved banners against Prodi as well as Bush.
The protesters included Senator Franca Rame, the wife of Nobel-prize winning playwright Dario Fo, as well as a number of Americans.
Some of them travelled to Rome specifically to protest against Bush and the war in Iraq.
BUSH SAYS DEMOS GOOD FOR DEMOCRACY.
Bush said on Saturday that the protests showed the democratic system at work.
White House spokeswoman Dana Perino reported the president as saying that "freedom of expression reinforces democracy".
Perino said Bush "is strongly convinced that you cannot have a democratic society if people are not allowed to freely express their opinions: this is the underlying force behind democracy and what keeps a country free".
While Prodi made no comment on the protests, his predecessor Silvio Berlusconi, who now heads the opposition, said the march was "inconceivable".
"This wouldn't have happened if I were still at the head of government," said the 70-year-old billionaire media mogul, who was a staunch ally of Bush during his five years in power.
"And if any of my MPs had wanted to demonstrate, then I would have told them to make a choice between supporting the government or taking to the streets," he added.
Some 10,000 police were deployed in the capital for Bush's visit, which ends on Sunday morning.
Helicopters hovered over the city for most of Saturday while Bush held talks with Prodi and Italian President Giorgio Napolitano and visited the Vatican for an audience with Pope Benedict XVI.
