Tens of thousands of people gathered in the Ukrainian capital on November 24 -- the second such demonstration in three days -- to protest the government's decision last week to suspend talks on a landmark Association Agreement with the European Union.
Police reported firing tear gas at a group of protesters that authorities said was trying to break through a cordon around the government building.
Separately, a smoke bomb was thrown near the building of the presidential administration.
Police said that some 23,000 people turned out for the rally, but the opposition said 100,000 attended.
Protesters, many of them waving European Union and opposition party flags, marched through the center of the city chanting pro-Western and antigovernment slogans.
Chants of "Ukraine is Europe!" resounded from the mass of protesters.
Opposition party Batkivshchyna said in a statement that about 100,000 people took part in the rally. That figure could not be independently verified.
One demonstrator told RFE/RL's Ukrainian Service that the country's future is together with Europe.
"I came here to support Ukraine's aspiration to join Europe. That's all," the protester said. "Our future is only in Europe."
There was also a smaller antigovernment rally in Kharkiv, where opposition leader and ailing former Prime Minister Yulia Tymoshenko is serving a jail term on charges that rights activists and many Western governments say are politically motivated.
LIVE BLOG (in Ukrainian): Follow RFE/RL Ukrainian Service's continuing coverage of the demonstration in Kyiv
In the capital, supporters of the ruling Party of Regions organized a smaller, rival rally nearby. Some 10,000 people reportedly attended that rally.
One government supporter, Anton, said the EU shouldn't try to impose its rules on Ukraine.
"We came here to let Europe know that we want to join it but we do not want them to set strict conditions for us," the demonstrator said. "We want to build Europe in Ukraine, we know how to do it, we want people here to have jobs."
WATCH: Tens of thousands turned up for the pro-EU rally in the capital, Kyiv:
One of the EU's conditions for signing an Association Agreement was for Ukraine to allow Tymoshenko to receive medical treatment abroad.
Ukrainian lawmakers couldn't agree on that on November 21 and the Party of Regions-led government later announced the suspension of preparations for signing the Association Agreement at an EU summit in Vilnius.
EU officials had hoped to sign the accord with Kyiv at the EU's Eastern Partnership summit on November 28-29.
The EU said it was disappointed with the Ukrainian government's decision but remained committed to the people of Ukraine.
Russia has in recent weeks put intense pressure on Ukraine to scrap plans for EU integration, urging it instead to join the Customs Union -- a trade bloc of Russia, Kazakhstan, and Belarus.
The release of Tymoshenko whose conviction on charges of abuse of office is seen by Brussels as "selective justice" was one of the key conditions for signing an accord with Kyiv.
The opposition accuses President Viktor Yanukovych of seeking to keep Tymoshenko out of politics ahead of a presidential election due in 2015.
Protests were also staged in the western city of Lviv, and in Sevastopol, a city on the Crimean peninsula where the Russian fleet is based.
Swedish Foreign Minister Carl Bildt offered via Twitter that the "massive size" of the Ukrainian demonstration on November 24 "sends a signal":
No doubting the massive size of the pro-EU manifestation in Kiev today. Sends a signal. pic.twitter.com/do9anwe7BA— Carl Bildt (@carlbildt) November 24, 2013