By Natalia Zinets and Richard Balmforth
KIEV (Reuters) - Ukrainian opposition leaders called on Sunday for President Viktor Yanukovich and his government to resign at a huge pro-Europe rally of about 350,000 people, marred by violent clashes between protesters and riot police.
In the biggest protest in the capital Kiev since the "Orange Revolution" of nine years ago, opposition leaders denounced Yanukovich for walking away from a pact offered by the European Union and swinging trade policy back toward Russia.
"They stole the dream," heavyweight boxer-turned-opposition politician Vitaly Klitschko told crowds on Independence Square.
The opposition urged people to demonstrate peacefully and avoid being provoked by the authorities into antagonizing police.
But violence erupted nonetheless with police using tear gas and stun grenades near the presidential administration. Police later clashed with a group of masked protesters trying to pull down a monument to Soviet state founder Vladimir Lenin.
Tens of thousands of Ukrainians swept on to Kiev's Independence Square, chanting "Down with the Gang!" and waving the Ukrainian flag and EU standard, in condemnation of Yanukovich's U-turn away from the EU.
The scale of Sunday's protest, which also marked the anniversary of Ukraine's 1991 referendum on independence, evoked memories of the 2004-5 Orange Revolution which overturned the established political order and doomed Yanukovich's first bid for the presidency.
After months of pressure from Russia, Yanukovich last month suddenly back-pedaled from signing a deal, long in the making, on closer relations with the EU in favor of renewed economic dialogue with Moscow, Ukraine's former Soviet master.
TV footage showed Berkut (special forces) police striking people on the legs and body with batons or kicking them as they lay on the ground.
Kiev's medical authorities said 112 people were given first aid treatment for injuries on Saturday, 42 of whom were kept in hospital. Police said 100 officers had been injured in the violence during the day.
An online TV station, called Public TV, on Sunday listed 29 journalists, mainly cameramen and photographers, who had suffered at the hands of police while covering the weekend events in Kiev.
At least 12 of these had been beaten by riot police. Those hurt included a Reuters cameraman who was beaten on the arms and whose camera was destroyed.
"If this government does not want to fulfill the will of the people, then there will be no such government, there will be no such president. There will be a new government and a new president," declared Klitschko, a contender for the next presidential election due in 2015.
Far-right nationalist leader Oleh Tyahnybok, also of the opposition, called for workers' support. "From this day, we are starting a strike," he declared. Backing for a national strike may indicate the protests' staying power in the coming days.
CITY HALL BREAK IN
Protesters broke into Kiev's City Hall during the demonstrations, and members of Tyahnybok's Svoboda (Freedom) party occupied the building, walking from room to room and holding meetings with followers of former economy minister Arseny Yatsenyuk's Batkivshchyna (Fatherland) Party.
All three opposition leaders also occupied a trade union building, turning it into a temporary headquarters.
The incident near the presidential administration building began after a group of protesters apparently commandeered a building excavator and charged police lines.
Klitschko and other opposition leaders said the apparent move to storm Yanukovich's main working headquarters was a clumsy stage-managed maneuver by the authorities to justify a crackdown.
"We know that the president wants to ... declare a state of emergency in the country," Yatsenyuk told reporters.
Police fired tear gas and stun grenades to push the protesters back. TV footage later showed riot police leading individuals onto the forecourt of the presidential building and kicking and beating them on the ground.
Similar scenes occurred later at night near Lenin's statue.
Yanukovich's U-turn has highlighted an old East-West tug-of-war over Ukraine, which is the cradle of eastern Slavic tradition while today sharing borders with four EU countries.
Yanukovich, a native Russian speaker, represents a constituency in the industrial east that has close cultural and linguistic kinship with Russia. In Ukrainian-speaking areas, particularly in the west, people have a more Western outlook.
Yanukovich says he has taken only a strategic pause in moves closer to Europe but the opposition accuses him of doing a deal with Russia that will harm national sovereignty.
It was hard to say whether Sunday's rally had weakened Yanukovich's grip on power or whether opposition leaders were now exerting real pressure on him. A gauge of their support will come from how much the strike call is heeded.
Serhiy Lyovochkin, Yanukovich's powerful chief of staff, was reported by Interfax news agency to have submitted his resignation, but there was no official confirmation of this.
Yanukovich is due to go off on a four-day trip to China from Tuesday and will, some time after that, travel to Russia to lay the ground for new economic ties with Moscow, his prime minister said on Saturday.
Trying to defuse tensions before Sunday's rally, Yanukovich said he would do everything in his power to speed up moves toward the EU. But he repeated the need to balance European integration with national interests.
"I want my children to live in a country where they don't beat young people," said protester Andrey, 33, the manager of a large company, who declined to give his surname for fear of reprisals.
Iryna Lukyanenko, a 19-year-old student, said more than just relations with the EU were at stake. "After Saturday, when the protest was broken up, I thought I should come today to defend our rights. We are no longer talking about 'euro-integration', but our rights."
(Additional reporting by Pavel Polityuk; Writing by Richard Balmforth and Thomas Grove; Editing by Philip Barbara)