A pro-European integration protester walks between a riot police line in Kiev Dec 11, 2013. Ukrainian riot police reoccupied part of the square in central Kiev on Wednesday where protesters have been demonstrating against the government's decision to pull out of negotiations on a trade pact with the European Union and rebuild economic ties with Russia. [Photo/Agencies]

KIEV - Ukrainian riot police early on Wednesday poured into Kiev's Independence Square, confronting opposition leaders and protesters demonstrating against a government decision to rebuild trade ties with Russia rather than move closer to the European Union.

Black-helmeted police entered the square in the heart of the snowbound capital where the demonstrators have been camping out for the past 10 days, tearing down barricades erected by the protesters.

Clashes broke out as police tried to detain some of the protesters. On one of the main streets leading into the square, a large group of protesters in hard hats, appointed to protect the demonstrators, held back a separate unit of riot police.

In emotional scenes, priests intoned prayers from a stage on the square and urged police not to use violence. Ruslana, a Ukrainian singer, called from a loud hailer: "Do not hurt us!"

Some protesters held mobile phones in the air like candles and sang the national anthem, while church bells rang out from a cathedral about 2 km (about 1 mile) away as in times of danger centuries ago.

The police move came after President Viktor Yanukovich on Tuesday defended his decision to walk away from signing a major trade pact with the EU on November 21 and instead revive economic links with Russia, Ukraine's former Soviet master.

Despite their large numbers, police did not initially use force but simply herded back protesters against a stage that opposition leaders have used in recent weeks as a platform from which to call for the government's dismissal.

They made no attempt to take down the encampment of tents on the square.

But clashes between the protesters' security guards and police nonetheless broke out as police tried to make arrests. Interfax news agency said some people had been hurt but it could not give an exact tally.

The police action drew an immediate response from the United States, which on Tuesday sent US assistant secretary of state Victoria Nuland to Kiev, as well as the European Union.

"The United States expresses its disgust with the decision of Ukrainian authorities to meet the peaceful protest in Kyiv's Maidan Square with riot police, bulldozers, and batons, rather than with respect for democratic rights and human dignity," US Secretary of State John Kerry said in a statement.

"This response is neither acceptable nor does it befit a democracy," he said in a statement in which he urged the authorities to show restraint.

EU foreign policy chief Catherine Ashton, who is also in Kiev and was expecting to meet Yanukovich on Wednesday, said in a statement: "I observe with sadness that police use force to remove peaceful people from the centre of Kyiv. The authorities did not need to act under cover of night to engage with society by using police."