* Putin unleashes intervention to protect Russian citizens
* Russian forces seize Crimea, no sign of them elsewhere
* Pro-Russian demonstrators hoist Russian flag in south, east Ukraine
By Pavel Polityuk and Alissa de Carbonnel
KIEV/BALACLAVA, Ukraine, March 1 (Reuters) - Russian President Vladimir Putin secured his parliament's authority on Saturday to invade Ukraine after troops seized control of the Crimea peninsula and pro-Moscow demonstrators hoisted flags above government buildings in two eastern cities.
Putin's open assertion of the right to deploy troops in a country of 46 million people on the ramparts of central Europe creates the biggest direct confrontation between Russia and the West since the Cold War.
It followed days of warnings from U.S. President Barack Obama and other Western leaders that Russia must not intervene, and assurances from Moscow that it would not do so.
Putin swiftly secured unanimous approval from Russia's senate for the use of armed force on the territory of his neighbour, citing the need to protect Russian citizens, the same reason he gave for invading tiny Georgia in 2008.
Britain summoned the Russian ambassador. EU ministers were due to hold emergency talks. Czech President Milos Zeman recalled the Soviet invasion of Czechoslovakia in 1968.
Troops widely believed to belong to Moscow have already seized Crimea, an isolated peninsula in the Black Sea where Moscow has a large military presence in the headquarters of its Black Sea Fleet. The campaign there has been bloodless so far, with Kiev's new authorities powerless to intervene.
Scores were also hurt on Saturday in clashes between pro-Russian demonstrators and supporters of Kiev's new authorities in eastern cities - areas near the Russian frontier, where Moscow is staging war games on high alert.
Putin asked parliament to approve force "in connection with the extraordinary situation in Ukraine, the threat to the lives of citizens of the Russian Federation, our compatriots" and to protect the Black Sea Fleet in Crimea. The authorisation to use force in Ukraine would last "until the normalisation of the socio-political situation in that country".
The upper house swiftly delivered a unanimous yes vote, shown on live television.
FLAGS TORN DOWN
So far there has been no sign of Russian military action in Ukraine outside Crimea, the only part of Ukraine with a Russian ethnic majority, which has often voiced separatist aims.
The Kremlin has not yet openly confirmed that the troops that have seized Crimea are Russian. A Kremlin spokesman said Putin had not yet taken the decision to use force under the authorisation granted by the upper house, and still hoped to avoid further escalation.
However, tension also rose dramatically elsewhere, with big and occasionally violent demonstrations in eastern and southern cities, where most people, though ethnically Ukrainian, speak Russian, and many support deposed President Viktor Yanukovich and Moscow.
By nightfall, demonstrators had torn down Ukrainian flags and replaced them with Russian flags on government buildings in the cities of Kharkiv, Donetsk, Odessa and Dnipropetrovsk.
In Kharkiv, scores of people were wounded in clashes when thousands of pro-Russian activists stormed the regional government headquarters, and fought pitched battles with a smaller number of supporters of Ukraine's new authorities.
Pro-Russian demonstrators wielded axe handles and chains against those defending the building with plastic shields.
In Donetsk, Yanukovich's home region, lawmakers declared they were seeking a referendum on the region's status.
"We do not recognise the authorities in Kiev, they are not legitimate," protest leader Pavel Guberev thundered from a podium in Donetsk.
Thousands of followers, holding a giant Russian flag and chanting "Russia, Russia" marched to the government headquarters and replaced the Ukrainian flag with Russia's.
Coal miner Gennady Pavlov said Putin's declaration of the right to intervene was "right".
"It is time to put an end to this lawlessness. Russians are out brothers. I support the forces."
"WAR HAS ARRIVED"
The rapid pace of events has rattled the new leaders of the country, who took power of a nation on the verge of bankruptcy when Yanukovich fled Kiev last week after his police killed scores of anti-Russian protesters in Kiev. Ukraine's crisis began in November when Yanukovich, at Moscow's behest, abandoned a free trade pact with the EU for closer ties with Russia.
After Russia's announcement of its intervention, Ukraine's acting president, Oleksander Turchynov, called a meeting of his security chiefs. Vitaly Klitschko, another anti-Yanukovich leader, called for general mobilisation.
On Kiev's central Independence Square, where protesters camped out for months against Yanukovich, a World War Two film about Crimea was being shown on a giant screen, when Yuri Lutsenko, a former interior minister, interrupted it to announce: "War has arrived."
Hundreds of people descended on the square chanting "Glory to the heroes. Death to the occupiers."
In Crimea itself, the arrival of troops was cheered by the Russian majority. In the coastal town of Balaclava, where Russian-speaking troops in armoured vehicles with blacked-out number plates had encircled a small garrison of Ukrainian border guards, families posed for pictures with the soldiers. A wedding party honked its car horns.
"I want to live with Russia. I want to join Russia," said Alla Batura, a petite 71-year-old pensioner who has lived in Sevastopol for 50 years. "They are good lads...They are protecting us, so we feel safe."
But not everyone was reassured. Inna, 21, a clerk in a nearby shop who came out to stare at the APCs, said: "I am in shock. I don't understand what the hell this is... People say they came here to protect us. Who knows? ... All of our (Ukrainian) military are probably out at sea by now."
For many in Ukraine, the prospect of a military conflict chilled the blood.
"When a Slav fights another Slav, the result is devastating," said Natalia Kuharchuk, a Kiev accountant.
"God save us."