Thousands have taken to the streets of the Ukrainian capital Kyiv, calling on the government to resign after it failed to sign an Association Agreement with the European Union.
Estimates for the number of protesters -- who are calling for the president and government to resign and for early elections to be held -- in Kyiv varied from between 100,000 to 500,000 people.
There were clashes between police and protesters, with some demonstrators occupying city administration buildings.
The leader of Ukraine's opposition Svoboda party claimed members of his party occupied the mayor's building in Kyiv.
Svoboda leader Oleh Tyahnybok was addressing tens of thousands of demonstrators on Independence Square and said members of Svoboda "took the Kyiv administration building."
Svoboda party member Yuriy Levchenko also spoke to the crowd, saying members of the party broke into the building, "to have a place to warm up, to use the toilet, to have some rest.”
“We are not vandals,” he added.
Tyahnybok also called for a nationwide strike and urged demonstrators on the square to remain there until their demands are met.
Kyiv police spokeswoman Olga Bilyk confirmed the mayor's office was occupied and said, "Law enforcement officials are now negotiating with those who took control of the mayor's office."
Yury Lutsenko, a former interior minister and now head of the opposition Third Ukrainian Republic, pointed to the large number of people involved in demonstrations and said, "This is already not a meeting or an action. This is a revolution."
Heavy-weight boxing champion and leader of the opposition Udar party Vitali Klitschko also addressed the crowd and appealed for demonstrators not to give in to provocation.
The opposition has claimed there are groups trying to infiltrate their demonstrations and provoke violence.
Medical authorities say 53 people have been hurt, mostly with head injuries or facial burns caused by tear gas. Fourteen of those people were admitted to hospitals.
The police have said that around 100 of their officers have been hurt with some 35 of them admitted to the hospital.
U.S. State Department spokeswoman Jen Psaki called on the Ukrainian government to respect the people's right to free expression and proclaimed that "violence and intimidation should have no place in today's Ukraine."
A joint statement issued by Polish Foreign Minister Radek Sikorski and his Swedish counterpart Carl Bildt -- two leaders spearheading the EU's expansion east -- also expressed support for the rallies.
"We are delighted that so many Ukrainians are braving the cold to protest their president's abrupt decision to withdraw from signing the Association Agreement with the European Union," Sikorski and Bildt said in their statement.
Demonstrators and police clashed outside the presidential building.
Protesters attempted to break down police barricades with a tractor, with police responding with stun grenades and tear gas.
The stand-off continued as darkness fell.
The Russian news agency ITAR-TASS reported a group of demonstrators has taken over the Trade Union House in Kyiv.
Demonstrations were also reported in other Ukrainian cities.
Ukrainian President Viktor Yanukovych has reportedly held an urgent meeting with the country's security council secretary, interior minister, the head of the Kyiv city administration, and other officials at his residence outside Kyiv.
Interfax reported that Serhiy Lyovochkin, Yanukovych's chief of staff, had submitted his resignation, although there was no official confirmation of this.
Despite the developing crisis at home, reports say Yanukovych plans to go ahead with a state visit to China later in the week.
Prime Minister Mykola Azarov said Yanukovych would also travel to Russia later this month to reach an agreement on normalizing trade ties.
Based on reporting by Reuters and Interfax