More than 100,000 demonstrators chased away police to rally in the center of Ukraine's capital on Sunday, defying a government ban on protests on Independence Square, in the biggest show of anger over the president's refusal to sign an agreement with the European Union.

Thousands of demonstrators tried to storm the nearby presidential administration building, but were driven back by riot police using tear gas and flash grenades, which produce a loud bang but are not intended to cause injury. The standoff continued, with more demonstrators arriving.

The protest was led by prominent opposition politicians, who demanded that President Viktor Yanukovych and his government resign. They also called for a nationwide strike and for tents to be set up to allow demonstrators to remain on the square around the clock.

"Our plan is clear: It's not a demonstration, it's not a reaction. It's a revolution," said Yuriy Lutsenko, speaking from the top of a bus.

Chants of "revolution" resounded across a sea of yellow and blue Ukrainian and EU flags on the square, where the government had prohibited rallies starting Sunday.

The crowd was by far the largest since the protests began more than a week ago. Many of the demonstrators had traveled to Kiev from western Ukraine, where pro-EU sentiment is particularly strong.

"We are furious," said Mykola Sapronov, a 62-year-old retired businessman. "The leaders must resign. We want Europe and freedom."

Protests have been held daily in Kiev since Yanukovych backed away from an agreement that would have established free trade and deepened political cooperation between Ukraine and the EU. He justified the decision by saying that Ukraine could not afford to break trade ties with Russia.