GWEN IFILL: There was deadly unrest in Eastern Ukraine today, as pro-Russia insurgents attacked a military checkpoint, killing 16 soldiers. In the town of Lysychansk, separatist rebels clashed with Ukrainian forces and exchanged mortar rounds and gunfire.

The country's acting prime minister accused Russia of escalating the conflict and trying to disrupt Sunday's election.

Chief foreign affairs correspondent Margaret Warner and her PBS NewsHour team is in Donetsk, where they witnessed the friction between separatists and election workers firsthand.

MARGARET WARNER: Donetsk region election official Evgeny Natsoyok was giving a tour this morning of district offices ransacked by pro-Russia separatist forces in recent days, when he responded to a jarring text message that his own headquarters office was being overrun that very moment.

Sure enough, armed men from the self-proclaimed Donetsk People's Republic, or the DPR, were seizing the building. They took computers and pulled election papers out of a safe. In a tense meeting with office head Ruslon Kudebiasev, they said they were there to prevent pre-ballot fraud leading up to Sunday's presidential vote.

MAN (through interpreter): How you can live with this?

MARGARET WARNER: One DPR squad leader, Judiev Vladimir, was unapologetic.

MAN (through interpreter): We are closing this polling station because we don't want these elections to happen.

MARGARET WARNER: Late today, we were told the DPR had arrested Ruslon and taken him away.

Unarmed police guarding the post put up no resistance and said they don't know how they can possibly secure the actual voting places this Sunday either.

KAPLIN SERGEY, Ukraine State Police (through interpreter): We are forbidden to say this, but we are the hostage of this situation. Can we defend anyone in this situation? Physically, we can't.

MARGARET WARNER: Separatists have now succeeded in closing more than half the election commission headquarters in the east, according to today's report by Russia's Interfax news agency.

A crestfallen Natsoyok said their operation is proving effective even among dedicated election staff, with many no longer showing up for work.

There's a lot at stake for all of Ukraine in Sunday's presidential election, nothing less than a legally elected government in Kiev that Russian President Vladimir Putin won't be able to so easily dismiss as illegitimate. And getting enough voters participating from these eastern regions along the Russian border is critical to that.

The separatists held a hastily arranged independence referendum here 10 days ago and say that tens of thousands of voters in the two eastern-most regions, Donetsk and Luhansk, had overwhelming voted to break away from Ukraine.

This Sunday's presidential vote to replace the rump government in Kiev is the opportunity for those who want to remain part of Ukraine to let their voices be heard, like pensioner Lebedeva Galina.

LEBEDEVA GALINA (through interpreter): I want to go to my desires. What is happening in my country now, it's a chaos, it's anarchy. I want to vote for a good chief, a good and honest man who will help people live better.

MARGARET WARNER: They're opposed by residents like Uli Bolshov, who say they voted for independence and will boycott Sunday.

MAN (through interpreter): I won't vote because I think this upcoming election is not legal.

MARGARET WARNER: That's the argument made by separatist forces occupying the Donetsk government building, like Denis Pushilin, chair of the new Donetsk People's Republic Council.

DENIS PUSHILIN, Chairman, Donetsk People's Republic Council (through interpreter): We believe it is incorrect for what is now our neighboring country to conduct presidential elections here. These elections are planned by the Kiev pseudo-authorities. But we don't consider it proper if they go forward.

MARGARET WARNER: But when pressed whether it was his men shutting down election offices and warning people to stay away from the polls, he dodged.

DENIS PUSHILIN (through interpreter): People come and ask, can we shut down this commission? And we don't particularly resist. We don't forbid them from doing this. We don't stop them. And they go and go and close them.

MARGARET WARNER: So men who come in in face masks and are armed, they're not working for you?

DENIS PUSHILIN (through interpreter): Well, I'm not ruling out that they may do this. But such orders were not given.

MARGARET WARNER: Whether ordered or not, the intimidation campaign appears to be having the intended effect on many voters here. Anatoliy Volkov, a young petroleum engineer, said though he thinks the DPR is illegitimate, he will stay away this Sunday.

ANATOLIY VOLKOV (through interpreter): I will not go because I feel uneasy about that. It may be dangerous.

MARGARET WARNER: And why are you uneasy?

ANATOLIY VOLKOV (through interpreter): There are many threats and many rumors that there will be no election, and the Donetsk Republic has officially claimed, since we are not part of Ukraine any more, people shouldn't go vote.

MARGARET WARNER: Seventy-year-old retired chemistry professor Svetlana Antonova had originally wanted to be an election worker. Now she says she won't even vote because of the danger.

ANATOLIY VOLKOV (through interpreter): It's war here. And it is possible there will be people with automatic guns. I'm just a woman and I'm scared.

MARGARET WARNER: Journalist blogger Denis Kazanskiy, who insisted on talking out of sight in the woods, thinks enough East Ukrainians will stay away to cast doubts on the legitimacy of the whole election.

What will be the consequences for Ukraine if this election can't go forward, as you predict, here in the East?

DENIS KAZANSKIY, Journalist: I guess this territory will be like a gray zone, like a failed state. And it will be like such zone like Transnistria or Abkhazia between Ukraine and Russia, and it will be very bad for this region, because all the industry will be stopped. And it will be great economic crisis here.

MARGARET WARNER: The man appointed governor by Kiev to calm the Donetsk region is billionaire industrialist Sergey Taruta, who we met in March in the building now occupied by separatists. Last night, a group of them were feeding an outdoor campfire with Ukrainian presidential election flyers.

We found Taruta last night instead holed up in a local hotel. He said he expects more trouble in the days ahead.

GOV. SERGIY TARUTA, Donetsk, Ukraine (through interpreter): They will implement various scenarios. They will frighten people who come to the polling stations. They will threaten them. They will stage provocations so people won't come out, so that people will be afraid and stay away and not cast their vote.

MARGARET WARNER: He concedes two separatist-held cities in his region, Slavyansk and Kramatorsk, won't be able to hold voting inside the city limits.

GOV. SERGEY TARUTA (through interpreter): In the rest of the cities, there are risks. But we will try to facilitate the vote in the rest of these areas.

MARGARET WARNER: But you can't protect 2,400 polling stations.

GOV. SERGEY TARUTA (through interpreter): Yes, it's difficult, but we're obligated to do so. We have no other option. We will do everything possible to facilitate the elections.

MARGARET WARNER: Taruta does have some muscle backing him up. Ukrainian military units sent by Kiev has fanned out across the east with checkpoints like this one and targeted operations to narrow the separatist areas of control.

And DPR leader Denis Pushilin seemed to be feeling the pressure last night.

DENIS PUSHILIN (through interpreter): The Kiev junta are facilitating criminal actions against our co-citizens. Our people are dying every day. Right now, we are in a civil war. On our territory, there are occupying forces.

MARGARET WARNER: Yet, even if he and his allies manage to pull off the vote, Taruta fears the DPR won't give up unless Russian President Vladimir Putin gives a clear and convincing order.

DENIS PUSHILIN (through interpreter): No, they will attempt to continue existing regardless of the elections. So after the vote, it will be necessary to hold a dialogue with them and call on them to give up their weapons and to respond to the demands of the protesters.

MARGARET WARNER: Pushilin says a dialogue is a nonstarter.

DENIS PUSHILIN (through interpreter): The elections will not in any way revolve the situation. The people's republic is already proclaimed. No one trusts Kiev here in this land.

MARGARET WARNER: The prospect of continued chaos and division dismays retired professor Antonova.

SVETLANA ANTONOVA (through interpreter): I am half-Russian and half-Ukrainian. I can't split myself into pieces. I don't like when Russians say bad things about Ukrainians or when Ukrainians say bad things about Russians. I want a president who will quiet such nationalistic conflicts. We need someone who can solve this conflict with wisdom.

MARGARET WARNER: Right now, here in Eastern Ukraine, wisdom seems to be in short supply.