An estimated 2,000 police in helmets and riot gear surrounded the demonstrators on Independence Square. There were several small scuffles with police, but no major clashes into the evening.
Protests have been held in Kiev for the past week since President Viktor Yanukovych backed away from the agreement. Across town on Friday, another rally brought out thousands of people in support of the decision.
It was unclear how long the protests against Yanukovych would be allowed to continue. The square has symbolic weight as the iconic epicenter of the 2004 mass protests known as the Orange Revolution, which forced the rerun of a fraud-tainted presidential election. Yanukovych, the focus of those protests, is unlikely to risk allowing another such huge demonstration of discontent.
Opposition leaders who spoke to the crowd called for protests to grow.
"We will conduct massive protest actions in all of Ukraine. They must witness our strength," declared Arseniy Yatsenyuk, an ally of Yanukovych's arch-foe, the jailed former premier Yulia Tymoshenko. He said the opposition "will do everything" to try to force a presidential election before its scheduled date in 2015.
"Our right to live in a European country was stolen," world boxing champion and prominent opposition figure Vitaly Klitschko said.
At the end of the speeches, protest leaders called for a new mass gathering on Sunday. Tents housing some protesters have been on the square for days.
Many protesters holding Ukrainian and EU flags tore pictures of Yanukovych, who ditched the free trade pact with the EU at Friday's summit in Vilnius.
Yanukovych abruptly changed course for integration with the EU last week when his government announced it was suspending preparations for signing the agreement. The move angered many in Ukraine, where nearly half the population of around 45 million favors closer ties with the EU.
Yanukovych argued that Ukraine can't afford to sacrifice trade with Russia, which has tried to block the deal by banning some of Ukraine's imports and threatening more trade sanctions.
"Millions of Ukrainians don't want to return to the Soviet past," said Olga Shukshina, a 46-year-old doctor from Lviv, near the border with Poland.
The demonstrations in Kiev revived memories of the 2004 Orange Revolution, which overturned Yanukovych's fraud-tainted victory and helped bring a pro-Western president to power.
On Friday afternoon, about 10,000 Yanukovych supporters rallied at another central square just a few hundred meters (yards) from the site of the protests to voice support of his move.
"We will go bankrupt without Russia and what shall we eat then? European slogans?" 40-year-old Pyotr Novodkov said.
