By Damien McElroy, Donetsk, Yekaterina Kravtsova in Moscow and Roland Oliphant in Simferopol
Ukraine's military mobilised to thwart an apparent Russian advance for the first time on Saturday night as Kiev sent paratroops to defend a gas facility near Crimea with tensions high in the hours leading up to Sunday's independence referendum.
The foreign ministry in Kiev denounced an "invasion" by Russia's forces into its mainland, when 80 Russian troops backed by four helicopter gunships and armoured vehicles seized the gas pumping station near the village of Strilkove on the Arabat spit, which lies inside the Kherson region just north of Crimean territory.
It was not immediately clear whether the Russian landing - which would be the first time Moscow has sent its occupying troops beyond the Crimean peninsula - was the beginning of an occupation or merely a probing exercise.
An unnamed Russian official said the incursion had been made to guard against "terrorist attacks."
Russia's foreign ministry claimed it was receiving "many requests" to protect people in Ukraine "These appeals will be considered," a statement said.
However the Kremlin was embarrassed by a massive anti-war protest outside its own walls.
Tens of thousands filled central Moscow to protest Russia's threats to invade Ukraine and annex Crimea in one of the largest demonstrations seen in recent years.
Holding Russian, Ukrainian and European Union flags, white flowers and flowers coloured in Ukrainian flag, the verdict on the Kremlin's Ukraine policy was damning.
"No war. Putin go to hell. Don't touch Ukraine," the marchers shouted. "Bring back Russian troops."
Protesters walked the city's boulevards shouting slogans against President Vladimir Putin who, they said, "occupied Russia and now wants to occupy Ukraine".
"We want to live in a free country, we have never been so ashamed of our authorities as we are now," activists said from the stage at a rally organised after the demonstration.
Organisers, including leaders of Russia's opposition movement, said up to 70,000 people took part in the demonstration. Police, however, estimated that no more than 3,000 Muscovites came and dubbed the marchers "opponents of Crimea's reunion with Russia".
In New York, Russia vetoed a Western-backed resolution condemning the Crimea referendum at a UN Security Council emergency vote.
China, a habitual Kremlin ally in showdowns with the West, abstained out of its own antipathy to cross-border meddling.
Samantha Power, the US ambassador to the UN, said the showdown in New York had left Russia a virtual pariah.
"Russia is isolated, alone and wrong to block the resolution's passage," Mrs Power said. "As we speak, Russian armed forces are massing across Ukraine's eastern border. This is a sad and remarkable moment."
François Hollande, the president of France, warned on Saturday that Russian annexation of Crimea would trigger sanctions on military co-operation. Potential that could affect a contract to build two Mistral-class warships, floating military bases that carry helicopters, tanks and troops for Russia.
William Hague, the Foreign Secretary, has called for a "firm and united" response from the European Union. He will join other EU foreign ministers on Monday for a meeting where they are expected to impose travel bans and asset freezes on between 120 and 130 powerful Russians.
Fears that Russia is laying the ground for a larger scale intervention in Ukraine that would effectively split the country into eastern and western zones has fuelled spiralling violence in Ukraine's Russian-speaking cities.
A wave of pro-Russian protests swept eastern Ukraine on Saturday after two people were killed in the early hours in the region's biggest city, Kharkiv.
Oleksander Turchinov, Ukraine's acting president, blamed Kremlin agents for the spiral of unrest turning into deadly clashes. "You know as well as we do who are organising mass protests in eastern Ukraine - it is Kremlin agents who are organising and funding them, who are causing people to be murdered," he said during a briefing to the opposition in parliament.
Ihor Baluta, the Kharkiv governor said "well-planned" provocation by pro-Russian protesters on a Ukrainian language centre had led to two of the attackers being killed in crossfire.
Crowds numbering in the thousands staged massive rallies in both Kharkiv and Donetsk in a Moscow-backed clamour for the right to a Crimea-style vote.
Beefed-up lines of security forces struggled to contain the Russia-speaking crowds as thousands surged against official buildings.
Waving a multitude of Russian flags and the orange and black ribbons of St George, the crowd converged on the Donetsk offices of the SBU state security organisation.
Crying "Putin is listening" and "We demand to join Russia", the mob smashed windows and started tearing down security grilles. It was only when a senior official came out to accept a petition and promise "demands would be met" that tension began to ease.
Regrouping on the mining city's Lenin square, the organisers seem satisfied with the show of force. "Tomorrow will be much bigger," vowed one of the main stage organisers.
Assumptions that the crisis can be contained are being rapidly revised. "On one level this can be seen as part of a game by regional billionaires who do not want to lose out to the people in power in Kiev," said Alex Ryabchyn, a development economist. "When the violence takes over it's not a game. Things can go very badly wrong."
Mourners at the funeral for Dmytro Chernyavskiy, a 22-year old activist who was killed on Thursday night, said they were losing the battle with the pro-Russian groups on the streets. "I saw Dmytro in convulsions as he lay dying. He had tried to protect us from the attack but they stabbed him in the heart," said Valery Kopachenski, an activist. "All we want is for Ukraine to stay one country. But for that to happen we need security to be able to show our feelings."
Kharkiv lies just 30 miles from the Russian border and Moscow has massed thousands of troop for exercises on the doorstop of the city of 1.4 million. While no one doubts the Russians could annex the city, defiant residents warn the move would rebound on the Russian leader.
"If Putin comes to Ukraine, that will be the end of him," said Vasily Golovin, a pensioner. "I can't read his thoughts, but if he thinks he can invade us, his game is up."
But the stakes in the war of nerves became clear by the circumstances surrounding Russia's latest incursion. Ukraine's defence ministry said the military scrambled aircraft and paratroops to repel an attempt by Russian forces to enter Arbatskaya Strelka, a long spit of land next to Crimea.
The Ukrainian ministry of defence initially said the Russians had withdrawn after paratroopers and elements of the army air corps were deployed to challenge the landing, but local news agencies later reported that Russian troops remained in the area.
The apparent incursion came as Russian troops continued to entrench their positions in Crimea, where voters will go to the polls today to vote in a referendum on unification with Russia.
Russian forces with artillery pieces were reported to be digging in near the northern town of Dzhonkoi in an apparently precaution against a counter attack from the direction of the Ukrainian mainland.
Polls will open at 8am and close twelve hours later today as voters are asked to chose one of two options. The plebiscite offers joining the Russian Federation as a new region, or reverting to Crimea's 1992 constitution, which would grant the region sweeping autonomy within Ukraine.
Sergei Aksyonov, the pro-Russian prime minister of Crimea, has said he is "certain" that voters will overwhelmingly back unification with Russia.
The Russian government has said it will "recognise" the outcome of the referendum, and a bill allowing break away regions to join Russia is set to be considered by the State Duma, the lower house of the Russian parliament, next week.
