KIEV (Reuters) - Ukraine's political opposition said on Saturday it would call a general strike to force the resignation of President Viktor Yanukovich's government after police used batons and stun grenades to break up pro-Europe protests.

Some 10,000 protesters regrouped in Kiev, flooding a square outside a church. They waved the blue and gold EU flag and chanted 'Revolution' and 'Down with the gang'.

Further rallies were planned for Sunday in the capital Kiev and other cities. Police promised there would be no repeat of the violence seen in the early hours of Saturday.

Helmeted police stormed an encampment in Kiev's Independence Square where protesters were singing and warming themselves by campfires early in the morning, the opposition said.

Tension had been building since Friday, when Yanukovich declined to sign a landmark pact with EU leaders at a summit in Lithuania, going back on a pledge to work towards integrating his ex-Soviet republic into the European mainstream.

Late on Friday live bands had played and the youthful crowd had brought almost a party spirit to the demonstration.

The protests evoked memories of the "Orange Revolution" of 2004-5, led by jailed former Prime Minister Yulia Tymoshenko, against election fraud which doomed Yanukovich's first bid for the presidency. But those protests were much larger and took place peacefully without police action.

Saturday's violence, in which police used stun grenades and batons, was unprecedented in Kiev.

Yanukovich, in an address carried on his website, said he was "deeply outraged" by the violent confrontation and injuries to people. He called for an immediate, objective investigation, but did not specifically blame police.

"EXCESSIVE FORCE"

TV footage showed police beating one young woman on the legs and kicking young men on the ground during the clashes.

EU officials, who had attended the EU Vilnius summit with Yanukovich, condemned what they called "excessive use of force".

"The unjustified use of force goes against the principles to which all participants of the Vilnius Summit...reaffirmed their adherence," European Union foreign policy chief Catherine Ashton and EU enlargement chief Stefan Fuele said in a statement.

The United States, which has backed Ukraine's EU integration efforts, also condemned "violence and intimidation".

A State Department statement said: "We continue to support the aspirations of the Ukrainian people to achieve a prosperous European democracy. European integration is the surest course to economic growth and strengthening Ukraine's democracy."

Yanukovich in Vilnius, justifying the decision to revert to Russia, Ukraine's former Soviet master, for economic aid, said the economic cost of meeting EU standards was too high.

His government says moves towards 'Eurointegration' have only paused, though the opposition says he is counting on Russia's help to secure a second term in office in an election in 2015.

PROTESTERS REGROUP

The Interior Ministry said the overnight violence had begun when protesters in Independence Square pelted police with trash. Police detained 35 people but quickly released them. There were no firm figures on how many people were hurt, but TV footage showed several with head wounds at the scene.

Ukraine's opposition condemned the crackdown and threatened a nationwide strike, while demanding the government's resignation and early parliamentary and presidential elections.

"We have taken a common decision to form a task force of national resistance and we have begun preparations for an all-Ukraine national strike," former economy minister Arseny Yatsenyuk, one of three opposition leaders, told journalists.

"RISE UP UKRAINE"

Nearly 10,000 protesters regrouped on Saturday, some of them holding candles, at Mikhailovska Square near a monastery where protesters had taken refuge from the police.

The Ukrainian Orthodox cathedral, where the faithful light candles before gilded icons of saints, was destroyed during the religious purges of Soviet dictator Josef Stalin and rebuilt after independence.

At the main gate, volunteers, some elderly, handed out food to the mostly young protesters as they compared stories of the night's violence.

Among chants of "Rise up Ukraine", former Interior Minister Yuri Lutsenko, a Tymoshenko ally who was released from jail earlier this year, encouraged a strike as early as Monday.

"From Monday, Kiev should be blocked and this strike will take away the power of the bandits," he said referring to corrupt politicians and powerful businessmen who Ukrainians say derive their wealth from their political ties with Yanukovich.

Kiev's police chief was reported as saying that he had given the order to send in riot police on Saturday but that there were no plans to do the same at Mikhailovska Square.

The protesters were mainly young supporters of the three main opposition parties, including Tymoshenko's, who are united in pressing for a westward shift in policy towards the EU.

Tymoshenko, seen by the EU as a political detainee, urged people "to rise up" against Yanukovich, in a letter read out by her daughter.

After breaking up the overnight protest, police cleared away anti-Yanukovich posters and political graffiti and took down banners and flags, including those of the EU.

Heavyweight boxing champion turned opposition politician Vitaly Klitschko said: "After the savagery we have seen on Independence Square we must send Yanukovich packing."

At least four people were beaten by police earlier on Friday, including a Reuters cameraman and a Reuters photographer, who was bloodied by blows to the head.

(Additional reporting by Pavel Polityuk in Kiev, Katya Golubkova in Moscow and Justyna Pawlak and Luke Baker in Brussels; Editing by Alistair Lyon,)