Italian riot police used tear gas Saturday against anti-Bush demonstrators who donned face masks in defiance of a police order and threw bottles and other objects, disrupting an anti-globalization demonstration that was ending in the capital's ancient center.

The violence broke out after U.S. President George W. Bush had concluded his official business for the day during a visit to Italy and had returned to the residence of the U.S. ambassador across town.

More than an hour into the clashes, police charged the demonstrators, pursuing them down alleyways to break up the crowd as helicopters circled overhead.

At least one protester was detained and taken away by police in plain clothes. An AP reporter saw at least one police officer injured as well as one demonstrator. The news agency ANSA reported three police injured.

The violence broke out as the protest march reached its destination at Piazza Navona, famed for its Bernini fountain and a favorite tourist gathering spot. Thousands of police had been deployed around the Colosseum, the downtown Piazza Venezia and other venues to guard against the violence and demonstrators had been warned not to cover their faces, carry sticks or other weapons.

Smoke filled a broad boulevard from which violent demonstrators lobbed objects at police taking cover in an alley. Nearby, demonstrators kicked in the window of a bank.

Masked demonstrators wearing black had infiltrated the otherwise peaceful protest and used it as cover to hurl objects at police. Other protesters later tried to stand between the violent elements and the police, putting up their arms to prevent further violence.

Organizers put the crowd in the main demonstration, which earlier had marched peacefully through the city behind a banner "No Bush, no war," and beating drums at some 150,000. The march stretched for about a kilometer (less than a mile.)

Protesters — many of whom had taken trains from cities in northern Italy — took aim at local and global issues, including the wars in Iraq and Afghanistan, as well as Italy's participation in the latter. Also targeted was Bush's environmental policies and the planned expansion of a U.S. military base in the northeastern city of Vicenza.

"The United States are a threat for their aggressive policies and for the cultural model that they try to export all around the world," said March Franchi of Perugia, who carried a banner that read: "God Smash America."

Besides the anti-globalization protest, far-left parties, including radical leftist politicians from Premier Romano Prodi's coalition, staged a separate demonstration in the central Piazza del Popolo, but turnout was below expectations.

The U.S. Embassy on Friday issued a warning to Americans to avoid the demonstrations or risk "becoming targets of opportunity."

White House spokeswoman Dana Perino said Bush had no problem with the demonstrations.

"That's what democracy is all about," Perino said. "He understands not everybody is going to agree with him."