UNITED NATIONS: Russia vetoed a Western-backed resolution condemning the Crimea referendum at a UN Security Council emergency vote Saturday but China abstained, isolating Moscow further on the Ukraine crisis.

The draft resolution, which says Sunday's referendum would have no validity, got 13 votes in the 15-member council. But it was rejected when permanent member Russia exercised its veto.

"Russia isolated, alone and wrong blocked the resolution's passage," US Ambassador Samantha Power told the Council at its seventh emergency session on Ukraine since the crisis began.

"This is a sad and remarkable moment," she said.

"As we speak, Russian armed forces are massing across Ukraine's eastern border," she added in a short speech.

China often backs Russia at the council, especially on Syria-related votes, and Western diplomats had seen its abstention as the best possible outcome from Saturday's vote.

When the Security Council ruled on a similar international crisis, between Russia and Georgia in 2008, Beijing abstained.

Saturday's emergency meeting was called at Washington's request and the resolution had been drafted by the United States in very measured terms so that it could be accepted by Beijing.

The resolution declared that the referendum on Crimea coming under Kremlin rule has "no validity and cannot form the basis for any alteration of the status of Crimea."

Meanwhile, tens of thousands have gathered in downtown Moscow in the largest anti-government demonstration since 2012, protesting against Sunday's Kremlin-backed referendum in Crimea on whether to break away from Ukraine and merge with Russia.

Demonstrators waved Russian and Ukrainian flags on Saturday while opposition activists — including the two members of the punk band Pussy Riot who served prison terms for an anti-government stunt — shouted "Say no to war!" and "Putin, go away!" from a stage.

Not far away near the Kremlin, several thousand people dressed in matching red costumes marched in formation to show their support for Russian intervention in the region.