Waving European Union flags, as well as the flags of Ukraine’s main opposition parties, a sea of demonstrators chanting “Ukraine is Europe!” marched from Shevchenko Park through Independence Square, the main site of Ukraine’s 2004 Orange Revolution, and on to European Square for a rally in front of Ukrainian House, a well-known building that houses a convention center.
Clusters of riot police officers were deployed along the route, drawing jeers from marchers but not interfering with the event.
“For young people, the future is brighter with Europe,” said Maria Lyskenko, 20, a student, who stood with her friend, Alyona Zorina, also 20, holding a sign that said, “Europe = Future Ukraine.”
Ms. Zorina said that President Viktor F. Yanukovich was acting out of selfishness and self-interest in deciding not to sign the agreements with the European Union. “He’s more interested in his own business dealings with Russia,” she said.
The political and trade accords, which had been in development for more than four years, had been scheduled to be signed later this week at a major conference in Vilnius, Lithuania. But, on Wednesday, the Ukrainian government announced that it had suspended its plans to complete the agreements.
On Sunday evening, several thousand protesters were engaged in a standoff with a phalanx of police outside the Cabinet of Ministers building. Some tussled with the officers, who were wearing gas masks, and tear gas was sprayed, but it was unclear whether the police or protesters had used it. At one point, a lit flare was thrown at the police in gas masks who were blocking access to the building, and a flagpole was knocked down.
The police reacted with restraint and there were no immediate reports of arrests. Some demonstrators, however, said that tension was building.
“The situation in society is boiling,” said Svyatoslav Tseholko, a television host and chief projects editor of Ukraine’s Channel 5, who attended the demonstration wearing a blue-and-yellow ribbon. “And we don’t know when is the time the top will pop off.”
Mr. Tseholko was urging demonstrators outside the Cabinet of Ministers building to remain calm and not resort to violence against the police, but he said that he understood the crowd’s anger over the decision to back away from the accords under pressure from Russia.
“It’s a sign that Ukraine can lose its statehood; it can lose its independence,” he said. “And if we lose our independence, we lose everything.”
In a sign of solidarity with the protesters, one of the country’s main news sites, Ukrainska Pravada — Ukrainian Truth — changed the name at the top of its home page to Evropeiska Pravada: European Truth.
Blue and yellow were the colors of the day, as some marchers wrapped themselves in the sky-blue and pale-yellow flag of Ukraine, while others wore the dark-blue flag with yellow stars of the European Union.
“Ukrainians want to live as Europe,” said Wladimir Klitschko, who was standing in at the rally for his brother, the boxing champion Vitali Klitschko, who is a member of Parliament and leads the political party Ukrainian Democratic Alliance for Reform. (Its acronym, UDAR, spells a word meaning “punch.”)
“I am confident that together we will get there,” Wladimir Klitschko said. “And I am certain that we will win.”
Ukrainian news agencies reported that Vitali Klitschko was out of the country to celebrate his daughter’s birthday and had tried to return for the protest, but his plane was unable to land in Kiev because of bad weather.
Some demonstrators said the protests would continue at least until Friday on the slim hope that Mr. Yanukovich will resurrect the agreements and sign them.