(Adds Lavrov, Kerry, Merkel, Hollande, edits)
* At least 26 killed, including 10 policemen
* EU leaders say urgently considering sanctions
* Yanukovich accuses opponents of attempt to seize power
* Hryvnia currency, Russian rouble fall
* Protesters gear for fresh action against police
By Richard Balmforth and Natalia Zinets
KIEV, Feb 19 (Reuters) - Ukrainian President Viktor Yanukovich accused pro-European opposition leaders on Wednesday of trying to seize power by force after at least 26 people died in the worst violence since the former Soviet republic gained independence.
European Union leaders condemned what they called "the unjustified use of excessive force by the Ukrainian authorities" and said they were urgently preparing targeted sanctions against those responsible for the crackdown.
The White House urged Ukraine to pull back riot police, call a truce and talk to the opposition. But the Ukrainian security services said they were launching an "anti-terrorist operation" across the country after the seizure of government buildings, arms and ammunition dumps by "extremist groups".
Protesters have been occupying central Kiev for almost three months since Yanukovich spurned a far-reaching trade deal with the EU and accepted a $15-billion Russian bailout instead.
The sprawling nation of 46 million, with an ailing economy and endemic corruption, is the object of a geopolitical tug-of-war between Moscow and the West. That struggle was played out in hand-to-hand fighting through the night, lit by blazing barricades on Kiev's Independence Square, or Maidan. As dusk fell on Wednesday, protesters braced for more police action.
Russian President Vladimir Putin and Yanukovich spoke by telephone during the night and both denounced the events as an attempted coup, a Kremlin spokesman said.
Russian Foreign Minister Sergei Lavrov, visiting Kuwait, blamed radical activists for the bloodshed and said: "I cannot leave without mention the responsibility that lies with the West encouraging the opposition to act outside of the law."
Moscow announced on Monday it would resume stalled aid to Kiev, pledging a $2-billion cash injection hours before the crackdown began. However, the money has not yet arrived and the Kremlin spokesman would not say when it would be paid.
Ukraine's hryvnia currency, flirting with its lowest levels since the global credit crunch five years ago, weakened to more than 9 to the dollar for the second time this month.
After a night of petrol bombs and gunfire on Independence Square, black smoke billowed from a charred trade union building that protest organisers had used as a headquarters.
Security forces occupied about a third of the square - the part which lies closest to government offices and parliament - with protesters pouring in to reinforce their defences on the remainder of a plaza they have dubbed "Euro-Maidan".
In a statement posted online in the early hours, Yanukovich said he had refrained from using force during three months of unrest but was being pressed by "advisers" to take a harder line: "Without any mandate from the people, illegally and in breach of the constitution of Ukraine, these politicians - if I may use that term - have resorted to pogroms, arson and murder to try to seize power," the president said.
He declared Thursday a day of mourning for the dead. The state security service said it had opened an investigation into illegal attempts by "individual politicians" to seize power.
One opposition leader, former world champion boxer Vitaly Klitschko, walked out of a overnight meeting with Yanukovich, saying he could not negotiate while blood was being spilt.
When fighting subsided at dawn, the square resembled a battle-zone, the ground charred by Molotov cocktails. Helmeted young activists used pickaxes, and elderly women their bare hands, to dig up paving to stock as ammunition.
The Health Ministry said 26 people were killed in fighting in the capital, of whom 10 were police officers. A ministry official said 263 protesters were being treated for injuries and 342 police officers, mainly with gunshot wounds.
The interior ministry said five of the dead policemen were hit by identical sniper fire in the head or neck. Journalists saw some hardline protesters carrying guns at the barricades.
EU WEIGHS SANCTIONS
In Brussels, European Council President Herman Van Rompuy said the 28-nation EU, at an emergency meeting on Thursday, would impose sanctions on those blamed for the bloodshed.
"Our ministers in the Foreign Affairs Council will at their meeting tomorrow examine targeted measures, such as financial sanctions and visa restrictions against those responsible for violence and use of excessive force," he said in a statement.
U.S. Secretary of State John Kerry, on a visit to Paris, will reiterate that Washington is open to imposing sanctions on Ukraine, a U.S. official said.
The European Investment Bank, the EU's soft-loan arm, said it had frozen its activities in Ukraine due to the violence.
The leaders of Germany and France said after talks in Paris that sanctions were only part of an approach to promote a compromise leading to constitutional reform and elections.
"What is happening in Ukraine is unspeakable, unacceptable, intolerable," French President Francois Hollande told a joint news conference. German Chancellor Angela Merkel said targeted sanctions against Ukraine's leaders would show the EU was serious in pressing for a political solution. She made clear they were talking to all sides in the crisis, including Russia.
Diplomats cautioned that any sanctions would be largely symbolic, noting that similar Western measures had long failed to sway or unseat the rulers of Belarus or Zimbabwe.
At the Winter Olympics in Sochi, in neighbouring Russia, Ukrainian Olympic Committee chief and former world pole vault champion Sergey Bubka said he was shocked by the violence and called for an end to the bloodshed.
"There is no 'their' Ukraine, or 'your' Ukraine. It is OUR Ukraine," he said in a statement.
In staunchly pro-European western Ukraine, opponents of Yanukovich declared political autonomy after seizing regional administrative buildings in the city of Lviv overnight and forcing police to surrender. Protesters also took over regional authorities' headquarters in Ivano-Frankivsk, blocked a road to a border crossing into Poland and torched the main police station in the city of Ternopil.
Many in the west, parts of which were first ruled from Moscow in World War Two, view Yanukovich as a corrupt ally of Russia and of business oligarchs in the Russian-speaking east.
Amid a tense standoff in the central Kiev square, thousands of protesters, many masked and in combat fatigues, confronted police across makeshift barricades for a second straight day.
Priests intoned prayers from a stage while young protesters in hard-hats improvised forearm and knee pads to protect themselves against baton blows. Others prepared petrol bombs.
"They can come in their thousands but we will not give in. We simply don't have anywhere to go. We will stay until victory and will hold the Maidan until the end," said a 44-year-old from Ternopil who gave only his first name of Volodymyr.
Traffic entering Kiev were restricted and the capital's metro was closed to prevent protesters getting reinforcements.
Demonstrations erupted in November after Yanukovich bowed to Russian pressure and pulled out of a planned far-reaching association agreement with Brussels. Western powers urged him to turn back to the EU and the prospect of an IMF-supported economic recovery, while Russia accused them of meddling.
Ukraine has been rocked periodically by political turmoil since independence from the Soviet Union more than 22 years ago, but it has never experienced violence on this scale.
Monday's announcement of the $2 billion payment was seen as a signal that Russia believed Yanukovich had a plan to end the protests and that he had dropped any idea of bringing opposition leaders into government. Late last month, one leader of the opposition rejected an offer to become prime minister. (Additional reporting by Matt Robinson and Pavel Polityuk in Kiev, Sylvia Westall in Kuwait, Adrian Croft in Brussels and Karolos Grohmann in Sochi; Writing By Paul Taylor; Editing by Alastair Macdonald)