Ukraine's opposition leaders called for western sanctions to be imposed and urged demonstrators to further protest action after police used force to break up a demonstration on Saturday against the government's refusal to sign a trade deal with the EU.

Kiev's central Independence Square has been ringed by police to prevent a repeat of the rally, which saw up to 10,000 people waving flags, singing songs and demanding the resignation of President Viktor Yanukovych.

The protest was prompted by Yanukovych's confirmation on Friday that he had decided to turn his back on a landmark pact with the EU, instead keeping Ukraine closely aligned with Russia.

About 500 police officers descended on the square – the symbolic heart of the 2004 Orange Revolution against elections rigged in favour of Yanukovych, as well as Ukraine's 1990 anti-Soviet protests – at 4am on Saturday, attacking protesters with truncheons. Yanukovich said on Saturday he was "deeply outraged" by the events which led to violent confrontation between protesters and police. He called for an immediate investigation, though did not specifically blame the police for the incidents.

"I just can't believe it happened," said student Igor Mitrov, with a bandaged head and a bloodstained Ukrainian flag in his hands. Mitrov, 22 was among protesters regrouping in the grounds of Kiev's St Mikhailovsky monastery. "The police were beating the girls with rubber batons and we, the guys, were trying to defend them. But without success."

Yaroslava Fedorash, 20, from Lviv in western Ukraine, described how police surrounded and pushed protesters into a metro station. "We were not resisting: we were just singing the Ukrainian anthem," she said. "I saw a girl whose hand was broken and the ambulance took her from the site."

The police action has generated international outrage. Britain's envoy to Ukraine, Simon Smith, tweeted that he was "hugely disturbed this morning to see pictures of deplorable intimidatory violence". The prominent Ukrainian human rights campaigner Yevhen Zakharov said the attack on peaceful demonstrators was unprecedented.

Yanukovych's opponents called the police action an attempt to "intimidate" people and demanded the resignation of the country's most senior police officer, Vitaliy Zakharchenko. They called for a general strike and urged people to attend a rally on Sunday to demand that Yanukovych step down and western countries impose sanctions.

Yulia Tymoshenko, the jailed former prime minister and opposition leader, whose incarceration on corruption charges is suspected by the west of being part of a political vendetta, called on Ukrainians to "step up against dictatorship and violence of Yanukovych" in a letter to the nation read by her daughter.

Vitali Klitschko, world heavyweight boxing champion and leader of the opposition Udar, or "Punch", party, warned that the violence against protesters should not be excused. "If we forgive such actions today, tomorrow they will repeat, tomorrow and the day after tomorrow, and Ukraine will turn into a police state."

Ukraine's prime minister, Mykola Azarov, said on his Facebook page that he was "outraged and concerned" about the incident but did not have enough information to make a final judgment. Ukrainian media are reporting that Sergiy Liovochkin, the president's chief of staff, has resigned in protest over the violence.

Police claimed their officers had been responding to complaints from municipal workers who said the demonstrators had been preventing them from preparing the main square for Christmas. "The actions of Berkut [the riot police unit] started after the protesters began fighting back at the police, scattering them with rubbish, glasses, bottles and burning sticks," a police statement said.

Thirty-five people arrested in the square were released following opposition complaints. Antoliy Vershygora, the head of the Kiev ambulance service, said seven people had been treated in hospital and that a further 14 needed medical assistance.

It was revealed on Friday that Yanukovych would not be signing an EU association and free trade deal at a summit in Lithuania as he could not afford to sacrifice economic ties with Russia. His decision was condemned by the European commission president José Manuel Barroso as a Russian "veto".