* 20,000 men sought for National Guard
* Minister says Russian separatists "on tour" in ex-Soviet world
* PM says earth will "burn" under the feet of separatist leaders (Adds quotes from Parubiy)
By Natalia Zinets and Richard Balmforth
KIEV, March 16 (Reuters) - Ukraine's new leaders announced on Sunday a call-up to raise 20,000 men for a newly-created National Guard, accusing Russia of sending "touring" trouble-makers across the border to stir up separatism in the country.
Keeping tension high as pro-Russian Crimean leaders staged a referendum for the peninsula to secede to Russia, pro-Western Prime Minister Arseny Yatseniuk pledged to bring to justice all those trying to destroy Ukrainian independence "under the cover of Russian troops".
Earlier, acting defence minister Ihor Tenyukh said Russia was pressing ahead with a military build-up in Crimea in violation of an agreement covering basing rights for its Black Sea fleet which has been there since Soviet times.
Instead of an agreed limit of 12,500 Russian servicemen in Crimea, Moscow now had 22,000 there, he said. "This is a crude violation of the bilateral agreements and is proof that Russia has unlawfully brought its troops onto the territory of Crimea."
He later told journalists that the defence ministries in Kiev and Moscow had declared a truce until March 21 during which Russian forces, who have been arriving by boat and helicopter, would leave Ukrainian military facilities untouched.
Yatseniuk, just returned from a trip to the United States where he won expressions of moral support but no offers of weapons, urged Ukrainians to sign on for service with the embryonic National Guard.
"It (the National Guard) will have training centres, legal weapons will be distributed. You will have the opportunity to defend the country, with the forces of the National Guard and the security forces," he told a cabinet meeting.
The country's new defence chiefs admit that the army is in poor shape and outgunned and outnumbered by Russian forces. But Andriy Parubiy, head of the National Security and Defence Council, said: "We are trying as quickly as possible to upgrade our armed forces so that they can work effectively."
Yatseniuk had a harsh warning for separatist "ringleaders" he said had sewed division in the ex-Soviet republic and were trying to destroy Ukrainian independence "under the cover of Russian troops".
"We will find all of them - if it takes one year, two years - and bring them to justice and try them in Ukrainian and international courts. The ground will burn under their feet."
Interior Minister Arsen Avakov said half the strength of the National Guard was being sought within two weeks. "About 10,000 will be called up in the next 15 days," he said.
Apart from Crimea, tension is also smouldering in parts of the Russian-speaking industrialised east of the country near the border with Russia, with clashes between rival demonstrators which Moscow has seized on to support its case that ethnic Russians are being victimised in Ukraine.
Two pro-Russian demonstrators were killed in Kharkiv on Friday night and one pro-Ukrainian activist was killed in Donetsk the night before.
In Donetsk on Sunday, activists angry about the detention of a pro-Moscow local leader broke into the prosecutor-general's office and raised the Russian flag. About 500 of them later broke windows at the local headquarters of the state security service but were held back from entering the building.
In the Ukrainian port city of Odessa, west along the coast from Crimea, a pro-Russian march drew about 5,000 people, some of them carrying the Russian flag, Interfax reported. No clashes with rival demonstrators were reported.
Remarks by Russian President Vladimir Putin and other high-ranking Kremlin officials that Moscow reserves the right to protect its compatriots and Russian-speakers in Ukraine have aroused fears in Kiev that Russia could be preparing broader military action against Ukraine beyond just Crimea.
But Avakov poured scorn on this argument on Sunday. "These people are professional touring provocateurs from a neighbouring country," he said, saying they were the same people who had stirred up trouble at pro-Russian rallies in the Baltic states in 2009.
He said that Ukrainian border guards and some members of the national Guard had now virtually sealed off parts of the eastern border with Russia to prevent people "with doubtful intentions" coming in. "Every day we are detaining hundreds of these people," he said.
Parubiy said that the Kremlin had predicted a far bigger response from pro-Russian supporters on the ground with them seizing official state buildings in eight administrative regions in an operation called the Russian "spring".
"We can say now that this plan has collapsed ... The Ukrainian authorities did not allow them to seize a single state institution," Parubiy said on television.
(Additional reporting by Ron Popeski; Writing by Richard Balmforth)