The demonstrators are singing the national anthem and chanting "Ukraine is Europe" while waving EU and Ukrainian flags.
Protests have been held daily in Kiev for more than a week after President Viktor Yanukovych backed away from an agreement that would have established free trade and deepened political co-operation between Ukraine and the EU.
He justified the decision by saying Ukraine could not afford to break trade ties with Russia.
The EU agreement was to have been signed on Friday and since then the protests have gained strength. Many of those taking part in Sunday's protest came from western Ukraine, where pro-EU sentiment is particularly strong.
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