* Vote organised on ad hoc basis, ballot paper confusion
* West fears slide into civil war
* Streets in rebel cities barricaded with felled trees, tyres
* U.S. says no sign Russia pulling troops back from Ukraine border
By Matt Robinson and Alessandra Prentice
MARIUPOL/SLAVIANSK, Ukraine, May 11 (Reuters) - Ukrainian leader Oleksander Turchinov warned pro-Russian eastern regions they would be stepping into the abyss if they voted for self-rule on Sunday in a referendum that has raised Western fears of a slide to full civil war.
Barricades of tyres and scrap metal blocked streets in the port city of Mariupol and in Slaviansk, centres of an uprising that has unleashed the worst crisis between the West and Russia since the Cold War. There was a clash between army and rebels near Slaviansk late on Saturday, but fighting had largely abated.
For a vote on which so much hangs, the referendum in the regions of Luhansk and Donetsk, which has declared itself a "People's Republic", seems a decidedly ad hoc affair.
Ballot papers have been printed with no security provision and the meaning of the question - asking voters if they support state 'self-rule' for the People's Republic of Donetsk - is, perhaps deliberately, unclear.
Some see in it endorsement of autonomy within Ukraine, some a move to independence and others a nod to absorption by Russia in the wake of Crimea, which Moscow annexed in March.
Annexation is favoured by the more prominent rebels, but the ambiguity may reflect their fears a full break would not garner enough support.
The eastern uprising followed the toppling in February of Ukrainian President Viktor Yanukovich, whose pursuit of ties with Moscow stirred mass protests by pro-Western activists in Kiev.
Rebels seized government and police buildings in the east with very little resistance from security forces Kiev says had been undermined by years of neglect and corruption under successive leaders.
But in recent days, Ukrainian forces have been striking back with shows of force especially in Mariupol, an important industrial and shipping centre. The interior ministry said 20 rebels were killed in fighting on Friday, while hospitals put the number of dead at seven.
Russia denies Western and Ukrainian accusations it has fostered the rebellion. It portrays the Kiev government as hostage to violent anti-Russian nationalists, intent on rooting out the culture and language of the Russian-speaking east.
Acting president Turchinov is attempting to bring eastern political forces into a round table to discuss federal devolution, but says he will not deal with rebel leaders with blood on their hands.
He said secession from Ukraine "would be a step into the abyss for these regions... Those who stand for self-rule do not understand that it would mean complete destruction of the economy, social programmes and life in general for the majority of the population."
The loss of Ukraine's coal and steel belt, which accounts for some 16 percent of national GDP, would be a severe loss for Kiev.
U.S. "DISAPPOINTED" RUSSIA NOT USING ITS INFLUENCE
"A dreadful terror is in train with the support of a large part of the local population," Turchinov said. "It is a complex problem when a population deceived by (Russian) propaganda support terrorists."
Sunday's vote is going ahead despite a call by Russian President Vladimir Putin on Wednesday to postpone it - a move that briefly raised hopes for an easing of tension. Western leaders have accused Putin of destabilising Ukraine, and Washington criticised as "provocative" a trip he made to Crimea on Friday.
"We are disappointed that the Russian government has not used its influence to forestall these referenda since President Putin's suggestion on May 7 that they be postponed, when he also claimed that Russian forces were pulling back from the Ukrainian border," U.S. State Department spokeswoman Jen Psaki said in a statement on Saturday.
"Unfortunately, we still see no Russian military movement away from the border, and today Kremlin-backed social media and news stations encouraged residents of eastern Ukraine to vote tomorrow, one even offering instructions for polling stations in Moscow. Russian state media also continue to strongly back the referenda with no mention of Putin's call for postponement."
German Chancellor Angela Merkel and French President Francois Hollande said they would back further sanctions against Russia - including in energy, defence and financial services - if Ukraine's May 25 presidential election failed to go ahead because of disruption in the east. Psaki warned of moving quickly to impose "greater costs" on Russia in such an event.
Kiev sees the election as a way of securing full democratic legitimacy following Yanukovich's flight to Moscow.
In the rebel bastion of Slaviansk, most roads were blocked off with barricades of felled trees, tyres and old machinery.
Nikolai Mikolaichuk, a businessman who had paid out of his own pocket to buy materials to build voting booths, was overseeing their construction in the town's Lenin Palace of Culture.
"Everyone here is volunteering in their own way. No one's being paid... This referendum is for all of us," he said. "The Donetsk People's Republic is going to be its own country, with its own government, laws, even an Olympic team."
While most of the physical infrastructure appeared ready for the referendum, there appeared to be some confusion among residents about the choice they were being asked to make.
"What are the options? I'm for peace," ethnic Azeri market-seller Assan Assanov said, standing behind piles of potatoes and spring onions.
"That means you're for the republic," the woman at the neighbouring stall told him.
Vyacheslav Ponomaryov, hardline rebel mayor of Slaviansk, said he expected a 100 percent turnout in the referendum.
Voting is from 8 a.m. (0500GMT) to 10 p.m. (1900 GMT). The ballots will then be counted overnight and results sent to Donetsk in the morning. (Writing by Ralph Boulton; Additional reporting by Pavel Polityuk and Aleksandar Vasovic and Arshad Mohammed in Washington; Editing by Mark Trevelyan and Nick Macfie)