* Crimea's pro-Russian leaders want to join Russia
* West warns Russia not to annex Crimea
* Russian forces take over border guard posts
By Alissa de Carbonnel
SIMFEROPOL, Ukraine, March 9 (Reuters) - Russian forces tightened their grip on Crimea on Sunday despite a U.S. warning to Moscow that annexing the southern Ukrainian region would close the door to diplomacy in a tense East-West standoff.
Street violence flared in Sevastopol, home to Russia's Black Sea Fleet, when pro-Russian activists and Cossacks beat and kicked a group of Ukrainians at a meeting.
Russian forces' seizure of the Black Sea peninsula has been bloodless but tensions are mounting following the decision by pro-Russian groups that have taken over the regional parliament to make Crimea part of Russia.
The operation to seize Crimea began within days of Ukraine's pro-Russian President Viktor Yanukovich's flight from the country last month. Yanukovich was toppled after three months of demonstrations against a decision to spurn a free trade deal with the European Union for closer ties with Russia.
Ukrainian Prime Minister Arseny Yatseniuk will visit Washington this week for talks, a White House official said.
In the latest armed action, Russians took over a Ukrainian border post on the western edge of Crimea at around 6 a.m. (0400) GMT, trapping about 15 personnel inside, a border guard spokesman said, revising an earlier figure of 30.
The spokesman, Oleh Slobodyan, said Russian forces now controlled 11 border guard posts across Crimea, a former Russian territory that is home to Russia's Black Sea fleet and has an ethnic Russian majority.
In Sevastopol, several hundred people held a meeting demanding that Crimea become part of Russia, chanting: "Moscow is our capital".
Across town at a monument to Ukrainian poet Taras Shevchenko, violence flared at a meeting to commemorate the 200th anniversary of his birth, when pro-Russian activists and Cossacks attacked a small group of Ukrainians guarding the event and the police had to intervene.
Footage from the event showed a group of men violently kicking one of the Ukrainians as he lay on the ground and a Cossack repeatedly hit him with a long black leather whip.
In Simferopol, Crimea's main city, pro- and anti-Russian groups held rival rallies.
Several hundred opponents of Russian-backed plans for Crimea to secede gathered, carrying blue and yellow balloons the colour of the Ukrainian flag. The crowd sang the national anthem, twice, and an Orthodox Priest led prayers and a hymn.
Vladimir Kirichenko, 58, an engineer, opposed the regional parliament's plans for a vote this month on Crimea joining Russia. "I don't call this a referendum. It asks two practically identical questions: Are you for the secession of Ukraine or are you for the secession of Ukraine? So why would I go and vote?"
Around 2,000 Russian supporters gathered in Lenin Square, where there is a statue of the Soviet state founder, clapping along to nostalgic Soviet era songs being sung from the stage.
Alexander Liganov, 25 and jobless, said: "We have always been Russian, not Ukrainian. We support Putin."
President Vladimir Putin declared a week ago that Russia had the right to invade Ukraine to protect Russian citizens, and his parliament has voted to change the law to make it easier to annex territory inhabited by Russian speakers.
Speaking by telephone to German Chancellor Angela Merkel and British Prime Minister David Cameron on Sunday, Putin said steps taken by authorities in Crimea were "based on international law and aimed at guaranteeing the legitimate interests of the peninsula's population," the Kremlin said.
At a rally in the eastern city of Donetsk, home to many Russian speakers, presidential candidate Vitaly Klitschko, a former boxing champion, said Ukraine should not allowed to split apart amid bloodshed.
"The main task is to preserve the stability and independence of our country," he said.
The worst face-off with Moscow since the Cold War has left the West scrambling for a response, especially since the region's pro-Russia leadership declared Crimea part of Russia last week and announced a March 16 referendum to confirm it.
U.S. Secretary of State John Kerry, speaking to Russia's foreign minister for the fourth day in a row, told Sergei Lavrov on Saturday that Russia should exercise restraint.
"He made clear that continued military escalation and provocation in Crimea or elsewhere in Ukraine, along with steps to annex Crimea to Russia, would close any available space for diplomacy, and he urged utmost restraint," a U.S. official said.
A spokeswoman for the Organization for Security and Cooperation in Europe said military monitors from the pan-Europe watchdog had on Saturday been prevented for the third time in as many days from entering Crimea.
Shots were fired to turn back the mission of more than 40 unarmed observers, who have been invited by Kiev but lack permission from Crimea's pro-Russian authorities to cross the isthmus to the peninsula. No on was hurt.
Moscow denies that the Russian-speaking troops in Crimea are under its command, an assertion Washington dismisses as "Putin's fiction". Although they wear no insignia, the troops drive vehicles with Russian military plates.
A Reuters reporting team filmed a convoy of hundreds of Russian troops in about 50 trucks, accompanied by armoured vehicles and ambulances, which pulled into a military base north of Simferopol in broad daylight on Saturday.
The military standoff has remained bloodless, but troops on both sides spoke of increased agitation.
"The situation is changed. Tensions are much higher now. You have to go. You can't film here," said a Russian soldier carrying a heavy machine gun, his face covered except for his eyes, at a Ukrainian navy base in Novoozernoye.
Ukrainian troops are performing training exercises in their bases but there are no plans to send them to Crimea, Interfax news agency quoted acting Defence Minister Ihor Tenyukh as saying. Ukraine's military, with 130,000 troops, would be no match for Russia's. So far Kiev has held back from any action that might provoke a response.