Huge rally in Kiev in support of closer ties with EU
Some 200,000 people have rallied in the Ukrainian capital Kiev to protest against President Viktor Yanukovych's refusal to sign a landmark EU deal.
Mr Yanukovych backed out of signing the association agreement after months of negotiation, apparently under strong pressure from Russia.
He is to meet Russian President Vladimir Putin in Moscow on Tuesday.
The EU has put any new talks on the agreement on hold until there is a clear commitment to sign.
News agency estimates of the size of the crowd at Independence Square ranged from 150,000 to 300,000.
This is the latest in a series of demonstrations over the past few weeks by the opposition who see Ukraine's future as part of the EU rather than aligned with Russia.
The series of protests, the largest since Ukraine's 2004 Orange Revolution, is designed to push Mr Yanukovych to dismiss his government and call fresh elections, opposition leaders say.
Makeshift barriers around the perimeter of the main protest encampment on Independence Square have been strengthened following an attempt by special police to dismantle them earlier this week.
Opposition leaders have urged protesters to remain vigilant, fearing "provocateurs" could trigger clashes between rival demonstrators.
Yuri Lutsenko, a former interior minister and opposition politician, told the protesters on Independence Square they were fighting for independence.
"What is happening on the Maidan [square] today?" he said.
"It is an anti-colonial revolution. Above all, Ukrainians turned out to say to Moscow: 'We are no longer under your command, we are an independent country'."
A leading voice on US foreign policy, Republican Senator John McCain, also addressed the pro-EU protesters after a walkabout in the crowd.
"Ukraine will make Europe better and Europe will make Ukraine better," he told them.
"We are here to support your just cause, the sovereign right of Ukraine to determine its own destiny freely and independently. And the destiny you seek lies in Europe," he said.
One unnamed protester in the crowd told the Associated Press news agency: "I have come here to defend my rights.
"My little nine-month-old child is waiting for me at home, and I don't want the government to steal his future. And of course, we want to join the EU."
President Yanukovych, who was elected in 2010, retains strong support outside Kiev, in the south and east of Ukraine.
At a much smaller pro-government demonstration in a Kiev park on Sunday, demonstrator Maria Nikolayeva, 18, told Reuters news agency: "We are here to support the president and order. Yanukovych is our best prospect at the moment."
Another demonstrator, 43-year-old Sergei Antonovich, told AP why he rejected the association agreement.
"We'll become the slaves of Europe if we go into it," he said. "Look at history - only union with Russia can save Ukraine from catastrophe."
'Work on hold'
The EU's frustration at the Ukrainian president's position on reaching a deal with the bloc was evident in a tweet from EU Enlargement Commissioner Stefan Fuele announcing the suspension of talks.
"Work on hold," he tweeted, adding that he had told Deputy PM Serhiy Arbuzov that Ukraine needed to show a "clear commitment to sign".
Mr Yanukovych has said he fears the association and trade agreement will put at risk many enterprises dependent on trade with Russia.
The president - who says he eventually aims to sign the deal - has also admitted being under heavy pressure from Moscow, which wants Kiev to join a Russian-led customs union instead.
He has said the EU would need to provide at least 20bn euros (£17bn; $27bn) a year to upgrade Ukraine's economy.
Moscow has already put economic pressure on Ukraine, with customs delays and a ban on Ukrainian chocolates, and there are concerns it could escalate such measures if Kiev drew closer to Brussels.