* Ukraine's rebel east votes on secession
* What comes next? War, annexation or frozen conflict
* Not all support secession
* No minimum turnout, no Western observers (Adds economic effect of unrest)
By Maria Tsvetkova and Matt Robinson
DONETSK, Ukraine, May 7 (Reuters) - Behind the barricades of tyres and car bumpers, past the masked militants who hold Donetsk's filthy administration building, there is a man in a suit in a spotless office, working from an Apple iMac on a vote to dismember Ukraine.
Roman Lyagin, 33, head of this self-proclaimed republic's electoral commission, does not hide his distaste for the gunmen who seized control here a month ago, but he shares their aims.
"Every revolution accumulates its fair share of loons," he told Reuters. But, he said, "we simply cannot live any more within Ukraine. Ukraine has already de facto said goodbye to us."
If Kiev has not already lost its industrial east, it risks doing so on Sunday, when people in this steel and coal belt - many of them ethnic Russians or native Russian-speakers - vote in a 'Yes' or 'No' referendum on secession.
What comes next will decide whether this country of some 45 million people slides into civil war, or settles into a frozen conflict that could scupper any plans for integration into Western-led multinational institutions for years to come.
The self-proclaimed Donetsk People's Republic faces joining Georgia's Abkhazia and South Ossetia, Azerbaijan's Nagorno-Karabakh and Moldova's Transdniestria as would-be statelets, unrecognised and ignored almost the entire world over since being spawned by the 1991 breakup of the Soviet Union.
Kiev will reject the referendum result, but Russia's response is uncertain. It annexed Crimea after a similar plebiscite in March, held under the watch of a Russian invasion force, but has been careful not to show its hand ahead of Sunday's vote.
Despite recent opinion polls suggesting only a minority supports secession, Lyagin says the result is in no doubt.
The red-blue-black tricolour flag of the rebel republic already flies at the regional administration building in Donetsk, a well-tended city traversed by a long park dotted with open-air cafes and restaurants. There are nods to the old Soviet order such as a statue of Bolshevik leader Vladimir Lenin.
Beyond the city, large factories and mines loom over a rolling green landscape.
Western observers won't be attending the poll, their absence fueling accusations that it will be fixed. Nor will there be a minimum turnout for the result to stand. Neighbouring Luhansk region, on the Russian border, will also vote. Gunmen hold sway there too, stoking fears that not everyone's voice will be heard.
ODESSA DEATHS A WATERSHED
U.S. Secretary of State John Kerry dismissed the referendum as "contrived and bogus".
"We flatly reject this illegal effort to further divide Ukraine," he told a Washington news conference.
Interviews in towns and cities across the rebel zone reveal deep division among the more than three million people who Lyagin says are eligible to vote.
Reuters reporters have spoken to people who variously want greater autonomy within Ukraine, a federal arrangement, an independent statelet and annexation by Russia.
But opinions are hardening behind the need to distance themselves from Kiev one month into an uprising that entered a bloody new phase last week with the launch of a Ukrainian military offensive.
The deaths of dozens of pro-Russians in a burning building in the Black Sea port of Odessa on Friday during street clashes with pro-Ukrainians marked for many the point of no-return.
"Was that normal what happened?" asked Alexander Proskurin, a 25-year-old casino croupier in the town of Kramatorsk. "Joining Russia is a lesser evil than remaining within Ukraine."
But 54-year-old Nikolai, part of a crowd of people in the town blocking a road to prevent Ukrainian forces from advancing, said all he wanted was greater decentralisation of power so this mining region could enjoy more of the fruits of its labour - approximately a third of Ukraine's industrial output.
"Power should be here, not in Kiev," he said. "No one's saying we should join Russia, but Kiev won't negotiate. Instead of negotiating, they send in the army."
Analysts say the conflict is taking a heavy toll on the entire Ukrainian economy. In a Reuters survey, they forecast it will slide deeper into recession this year as the rebellion cripples activity in the east and scares off foreign investors.
The referendum threatens to further undermine a presidential election in Ukraine two weeks later.
The Western-backed government in Kiev hopes that poll can draw a line under months of turmoil since protests broke out in the capital over then president Viktor Yanukovich's decision to spurn closer ties with the European Union.
Lyagin used to be a consultant for Yanukovich's Party of Regions, but the burly president was ousted in late February after gunbattles in central Kiev.
His flight to Russia triggered an uprising in Crimea and then his native east that the Western-backed government says is being orchestrated by the Kremlin, a charge Russia denies.
"May 11 is not a referendum," said Petro Poroshenko, a Ukrainian oligarch, former government minister and leading presidential candidate. "It is a sham aimed at weakening the Ukrainian state and tearing it apart."
"People are afraid to go out on the streets in conditions of anti-terrorist operations, let alone go to vote."
Kiev is proposing its own referendum on decentralisation, but only once the fighting stops. "A referendum must not be carried out under the barrel of a gun," Andriy Senchenko, deputy chief of staff to acting Ukrainian President Oleksander Turchinov, said on Tuesday.
By then it may already be too late. Ukraine says its forces are tightening a noose around the rebel stronghold of Slaviansk, but progress appears patchy.
An advance into the city could trigger a bloodbath, with tens of thousands of Russian troops massed on the border some 160 km (100 miles) away.
"Kiev won't like the result, so there's going to be more bloodshed," said 58-year-old Slaviansk bus driver Pyotr Kosakhov. "They've shown what they're willing to do. People here don't want anything to do with Ukraine anymore."
Lyagin, in Donetsk, knows which way he will vote. He said: "We're already not in Ukraine." (Additional reporting by Pavel Polityuk and Natalia Zinets in Kiev and Alessandra Prentice in Slaviansk, Ukraine; Writing by Matt Robinson; Editing by Ralph Boulton)