Kiev, Ukraine (CNN) -- At least 100,000 protesters calling for the Ukrainian government's ouster packed Independence Square in Kiev on Sunday in a continued demonstration against the president's decision to not sign a landmark trade deal with the European Union.
Protesters are demanding new parliamentary and presidential elections.
"We will not leave the square until the government resigns," said Vitaliy Klitchko, one of the protest leaders.
Riot police lined up to protect the office of President Viktor Yanukovych, whose decision last month not to sign the deal sparked the public outrage.
Police officials told CNN that at least 15 officers were hospitalized after clashes with protesters, who at one point used a bulldozer to try to break through a barricade. Some protesters were seen setting objects on fire and throwing them at security forces. Others broke through a police barrier that was protecting a Christmas tree, climbed the tree and topped it with the Ukrainian flag.
Kiev city officials said 53 protesters asked for medical assistance.
Klitchko dismissed the excessive violence, saying it was a setup.
"We should not be provoked. There are a lot of provocateurs. We can change the power in a civilized way. Our rally in Kiev today is a peaceful protest only," he told people during a speech in the square.
At the heart of the protests is Ukraine's about-face after a year of insisting that it was intent on signing a historic political and trade agreement with the European Union.
The demonstrations began a week ago, just days after Yanukovych suspended talks with the EU.
The protesters, who have set up tents in Independence Square, are calling for a general strike.
All day, the crowd started singing the national anthem whenever there was a threat of violent outbursts -- a scene reminiscent of Ukraine's "Orange Revolution" of 2004, when millions joined in peaceful protest against alleged corruption and in defense of democracy.
Yanukovych is in a tight spot. Under severe economic pressure from Ukraine's giant neighbor, Russia, not to join the EU, he also was facing a key EU demand that he was unwilling to meet: freeing former Prime Minister Yulia Tymoshenko, his bitter political opponent.
Two years ago, she was found guilty of abuse of office in a Russian gas deal and sentenced to seven years in prison in a case widely seen as politically motivated. Her supporters say she needs to travel abroad for medical treatment.
The deal, the EU's "Eastern Partnership," was aimed at creating closer political and economic ties and fostering economic growth among the nations of Eastern Europe and the Caucasus, including Ukraine, Moldova, Georgia, Armenia, Azerbaijan and Belarus.
Earlier Sunday, Kiev's police chief Valeriy Koryak resigned after riot police used "excessive force" against anti-government protesters on Saturday, police said.
On Saturday, seven people were hospitalized and dozens arrested after riot police stepped in.
The United States condemned what it called "violence against protesters" in a statement posted online by the U.S. Embassy in Kiev.
Ukrainian Interior Minister Vitaliy Zakharchenko later apologized, saying riot police abused their power. He promised a thorough investigation.
But he also warned protesters via state television against improper behavior, saying "if there are calls for mass disturbances, then we will react to this harshly."
CNN's Zarifmo Aslamshoyeva and Boriana Milanova contributed to this report.