Several hundred thousand Ukrainians occupied a central square in the capital on Sunday, denouncing President Viktor Yanukovych’s decision to turn away from Europe and align this former Soviet republic with Russia.

As widespread protests continued for a third week, Mr. Yanukovych’s meeting on Friday with Russian President Vladimir Putin raised more fears that Ukraine is on the verge of entering a Russian-led customs union that critics say could end its sovereignty and place it back under Moscow’s rule. Authorities have said that police will not take action against peaceful demonstrators, but concerns persist that some opposition activists may be goaded into violence.

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