WASHINGTON (Reuters) - The United States expressed concern on Monday about the Turkish police's rough treatment of anti-government protesters, in a rebuke to a NATO ally that Washington has often held up as an example of a Muslim democracy.

"We are concerned by the reports of excessive use of force by police," U.S. Secretary of State John Kerry told reporters. "We obviously hope that there will be a full investigation of those incidents and full restraint from the police force."

Hundreds of protesters and police have been injured in demonstrations that have turned into the most violent rioting that Turkey has seen for decades. While Kerry carefully referred to "reports" of excessive force by the police, the State Department later said it believed this had taken place.

Turkey is overwhelmingly Muslim but has a strong secular and pro-Western tradition. It is one of the strongest U.S. allies in the Middle East and Prime Minister Tayyip Erdogan's Islamist-rooted government has often been promoted by U.S. officials as evidence that democracy and Islam are compatible.

The violence erupted after a demonstration on Friday to halt construction in a park in an Istanbul square grew into mass protests against a heavy-handed crackdown and what opponents call Erdogan's authoritarianism.

In central Ankara, demonstrators set up a barricade and lit a fire in a road on Monday, and Erdogan accused protesters of walking "arm-in-arm with terrorism."

Speaking at a news conference with the Polish foreign minister, Kerry said the United States strongly supports the right to peaceful protests and appealed to the government and protesters to avoid further provocation.

Erdogan's AK party has won three straight parliamentary elections and overseen an economic boom, increasing Turkey's influence in the region.

But opponents accuse him of growing increasingly authoritarian, muzzling the media, tightening his party's grip on state institutions and putting religion at the center of politics in violation of Turkey's secular constitution.

NO ILLUSIONS

Henri Barkey, a former State Department policy staffer who is now an international relations professor at Lehigh University in Pennsylvania, said the United States had long known that Erdogan's government was not "the pinnacle of democracy."

"The White House did not have any illusions about this government and this prime minister. But what they will be surprised about is how badly (Erdogan) mishandled the situation," he said. "This is the first time he has not succeeded in managing the crisis."

White House spokesman Jay Carney said U.S. officials believe the vast majority of the protesters in Turkey have been ordinary citizens engaged in peaceful demonstrations.

"We have concerns about some of the response, but we certainly expect the Turkish government to work through this," he said when asked about demonstrations and rioting.

In a commentary published on Monday, rights group Amnesty International said it had received reports from the Izmir Medical Association that 420 injured people had been admitted to hospitals in the western Turkish city over the last two days.

The medical association said many of their injuries were caused by the police using water cannon and firing teargas canisters at demonstrators, and that it was concerned that some people may have lost their eyesight as a result of the tactics.

"Water cannon and tear gas should not be used against peaceful protesters," said John Dalhuisen, Europe and Central Asia program director at Amnesty International. "We're particularly concerned about the use of tear gas in confined spaces where it represents a major threat to health."

Carney said the United States would continue to work with Turkey on Syria. Ankara is one of the main regional opponents of Syrian President Bashar al-Assad.

The civil war has spilled over into neighboring Turkey, which has stepped up security on its border after Syrian shells landed inside Turkish territory. Turkey is hosting some 400,000 Syrian refugees, about half inside camps and the rest living with relatives or in rented accommodation across Turkey.

(With additional reporting by Arshad Mohammed, Laura MacInnis, Rachelle Younglai and Mark Felsenthal; Editing by Alistair Belland Mohammad Zargham)