Thirty-three-year-old engineer Sergei said he would vote "Yes" to the question on the ballot paper, printed in Russian and Ukrainian: "Do you support the act of state self-rule of the Donetsk People's Republic?"
"We're all for the independence of the Donetsk republic," he said. "It means leaving behind that fascist, pro-American government (in Kiev), which brought no one any good."
Autonomy, independence, annexation
But in the same queue of voters, 54-year-old Irina, saw a 'yes' vote as endorsement of autonomy within Ukraine.
"I want Donetsk to have its own powers, some kind of autonomy, separate from Kiev. I'm not against a united Ukraine, but not under those people we did not choose, who seized power and are going to ruin the country."
At another Mariupol voting centre, officials moved some ballot boxes outside onto the street, placed against a wall.
Russia vs Ukraine: A fighting force comparison
The eastern rebellion began shortly after President Viktor Yanokovich fled to Russia in February under pressure from mass protests in Kiev by pro-Western activists. They took to the streets after he decided to discard a cooperation accord with the European Union in favor of closer ties with Moscow.
Some see a "Yes" vote as endorsement of autonomy within Ukraine, some as a move to independence and others as a nod to absorption by Russia.
Annexation is favored by the more prominent rebels, but the ambiguity may reflect their fears an explicit call for full "independence" might not have garnered the support they seek and could leave them in an exposed position towards Kiev.
Ukrainian leader Oleksander Turchinov has urged east Ukrainian political leaders to join a "Round Table" discussion on devolution of powers in Ukraine. But he has said he would not negotiate with "terrorists", a formulation meant to exclude most of the more prominent rebel leaders.
The Ukrainian Defense Ministry said rebels attacked their forces guarding a television tower on the outskirts of Slaviansk. One Ukrainian serviceman was wounded in fighting.
Sergei, a fighter speaking near an outer checkpoint, blamed Ukrainian forces for the clash.
"They are probably trying to put people off voting, but it won't work," he said.
Eastern rebels and the Kremlin regard the pro-European Kiev government that replaced Yanukovich as lacking legitimacy.
Putin marks Victory in Crimea
Kiev aims to banish such questions by holding national presidential election on May 25, but the West sees Russian efforts to disrupt them and threatened economic sanctions against Moscow over the weekend.
Sunday's vote went ahead in the important industrial region 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 destabilizing Ukraine, and Washington criticized as 'provocative' a trip he made to Crimea on Friday.
German Chancellor Angela Merkel and French President Francois Hollande said on Saturday they would back further sanctions against Russia if Ukraine's presidential election failed to go ahead because of disruption in the east.
Turchinov, who has ruled the referendum illegal and dismissed the allegations that the Kiev authorities are neo-fascists, told eastern regions on Saturday any move to secession would be "a step into the abyss".