Ukrainian riot police charged protesters occupying a central Kiev square early  Wednesday after the bloodiest day since the former Soviet republic, caught in a geopolitical struggle between Russia and the West, won its independence more than 22 years ago.

At least 18 people, including seven policemen, died Tuesday during hours of violence between security forces and civilians who have staged protests against President Viktor Yanukovich since last November.

Many were killed by gunshots and hundreds more were injured, with dozens of them in serious condition, police and opposition representatives said.

Despite Western demands for restraint and dialogue, the state security service first set a deadline for the demonstrators to end disorder or face "tough measures," and then the police advanced into Independence Square, the center of the protest campaign.

Protesters responded with gasoline bombs, fireworks and stones. Live television footage showed police throwing stun grenades at the protesters, separated from them by a line of burning tents, tires and wood.

Police steadily gained ground, but thousands of protesters held onto the center of the square, a Reuters cameraman reported.

Earlier, one of the opposition's leaders, Vitaly Klitschko, reacted defiantly, telling supporters in the square: "We will not leave here. This is an island of freedom. We will defend it."

The world-champion boxer-turned-politician later arrived at Yanukovich's office for late-night talks, Klitschko's spokeswoman said, but he and another opposition leader, former economy minister Arseny Yatsenyuk, were still waiting to see the president an hour later.

Earlier, the State Security Service, the SBU, in a joint statement with the interior ministry, signaled the government's intentions. "If by 6 p.m. the disturbances have not ended, we will be obliged to restore order by all means envisaged by law," it said.

The riot police moved in hours after Moscow gave Ukraine $2 billion in aid for its crippled economy, which it had been holding back to demand decisive action to crush the protests.

Nationwide demonstrations erupted after Yanukovich bowed to Russian pressure and pulled out of a planned far-reaching trade agreement with the European Union, deciding instead to accept a Kremlin bailout for the heavily indebted economy.

Western powers warned Yanukovich against trying to smash the pro-European demonstrations, urging him to turn back to Europe and the prospect of an IMF-supported economic recovery, while Russia accused them of meddling.

EU Enlargement Commissioner Stefan Fuele said he had spoken to Ukraine's acting prime minister, who had given assurances that the authorities would try to avoid using live firearms.

"For the sake of the Ukrainians and for the sake of the future of that country, I will pray that he is right," Fuele told a public event in Brussels.

In Washington, White House spokesman Jay Carney said the United States was appalled by the violence, and urged Yanukovich to restart a dialogue with opposition leaders. "Force will not resolve the crisis," Carney told a briefing.

Earlier on Tuesday, clashes raged for several hours outside the parliament building. A police spokeswoman gave a variety of reasons for the deaths, including gunshot wounds, a traffic accident and heart attacks. One protester died in a fire.

Right Sector, a militant far-right group, added to tensions by calling on people holding weapons to go to Independence Square — also known as Maidan — to protect it from security forces.

As protesters and police battled on the streets of Kiev, Russia called the escalation a "direct result of connivance by Western politicians and European structures that have shut their eyes . . . to the aggressive actions of radical forces."

EU foreign policy chief Catherine Ashton, who has tried to broker a power-sharing transition, urge Ukraine's leadership "to address the root causes of the crisis."

Germany's foreign minister, Frank-Walter Steinmeier, telephoned his Ukrainian counterpart to warn against sliding back into violence and to urge the government to keep working for a political solution.

"News of a fresh escalation of violence is alarming. We are shocked to hear of the dead and injured today," Steinmeier said, raising the possibility of EU sanctions against Ukrainian leaders.

"Those responsible for taking any decisions that lead to the further spilling of blood must know that the reserve Europe has shown in terms of personal sanctions will be reconsidered," he added.

Monday's $2 billion cash injection, a resumption of a $15 billion aid package, was seen as a signal that Russia believed Yanukovich had a plan to end the protests and had dropped any idea of bringing opposition leaders into government.