MARIUPOL/SLAVIANSK, Ukraine - May 10 (Reuters) - Ukrainian acting President Oleksander Turchinov told eastern regions gripped by a pro-Russian uprising that they would be courting catastrophe if they voted "yes" in a separatist referendum on Sunday.
Turchinov, who deems the vote in the Russian-speaking Donetsk and Luhansk regions illegal, urged the population to accept "round table" talks on greater autonomy. But, in reference to fighters who have seized police and government buildings, he said "terrorists" could not be included.
The vote, organised on a largely ad hoc basis with no clear control of authenticity of ballot papers or voter lists, could have serious consequences for Ukraine and relations between Moscow and the West. It risks turning isolated clashes into civil war.
"(Secession from Ukraine)...would be a step into the abyss for these regions," Turchinov said on his website. "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 in these regions."
The atmosphere in major cities across the region was tense though there were no reports of fighting in the morning.
In the port city of Mariupol, where between seven and 20 people were killed in fierce fighting on Friday, rebels blocked the streets with barricades of tyres, garbage containers and chairs. Smoke was still coming from the partially burnt-out administration building. There was no sign of Ukrainian forces.
The barricades were manned by a handful of pro-Russians, some with batons or clubs, wearing motorcycle helmets. No gunmen were visible. Video on the YouTube site showed an armoured car captured by rebels set on fire and ammunition exploding.
Throughout the city of Slaviansk, the most heavily defended separatist redoubt, streets were barricaded with tyres, furniture, cars and scrap iron.
In the city of Donetsk, rebels released several members of the Red Cross whom they held for seven hours, one having been beaten, a Red Cross official in Kiev said.
Western states prepared to step up pressure on Russia, whom they accuse of engineering the crisis to destabilise Ukraine. Russia denies involvement but voices support for insurgents it says are defending themselves against fascist Ukrainian forces.
German Chancellor Angela Merkel and French President Francois Hollande said that if May 25 national polls failed to go ahead because of the rebellion, this would further unsettle the country. In that case, they would be "ready to take further sanctions against Russia".
Western countries are expected to announce new economic sanctions over President Vladimir Putin's actions over Ukraine.
The European Union has so far imposed asset freezes and visa bans on 48 Russians and Ukrainians over Moscow's annexation of Crimea. EU diplomats say new sanctions will for the first time target companies.
The national polls are seen in Kiev as a way of establishing a fully legitimate, universally elected government following pro-Kremlin president Viktor Yanukovich's flight to Russia in February under pressure from pro-Western demonstrations.
In the largely rebel-controlled regions of Donetsk and Lugansk, which have declared a breakaway "People's Republic of Donetsk", preparations went ahead for Sunday's self-rule referendum, though there was widespread uncertainty about what the question on the ballot paper meant:
"Do you support the act of self-rule of the People's Republic of Donetsk?"
Some people interpret it as a vote for more local powers, some for broad autonomy within Ukraine, some for independence, others still as a step towards incorporation into Russia.
Vyacheslav Ponomaryov, the rebel mayor of the city of Slaviansk said he expected a 100 percent turnout. He set out conditions for talks with Kiev.
"The withdrawal of (Ukrainian) forces and exchange of prisoners," he told a news conference. "Only after fulfilment of these conditions would we be ready for talks. If the junta continues to retain its forces here, we will continue to fight."
Ponomaryov has been in the forefront of separatist activity, and Kiev could place him in the category of "terrorists" who would not be welcome at the round table for which Kiev is seeking international backing.
(Writing by Ralph Boulton; Reporting By Pavel Polityuk and Aleksandar Vasovic; Editing by Raissa Kasolowsky)