Preparations for the presidential elections in Ukraine have been plagued with meddling and threats as armed pro-Russia rebels are seizing voting materials in the country's east in a bid to halt the vote for the country's next leader.

"We are from the Donetsk People's Republic," shouted balaclava-clad men brandishing automatic weapons as they stormed into the meeting of election officials in Donetsk today. According to eyewitnesses the gunmen then ordered the terrified election commission members from Donetsk's district 44 to hand over the material they'd been preparing for this weekend's vote, including official stamps, voting lists, and ballot papers.

Following a dubious referendum on May 11 — held without international monitors and under the watchful eye of armed militia — the rebels declared that an overwhelming 90 percent in eastern Donetsk and Luhansk oblasts voted in favor of independence. With a supposed public mandate secured for the so-called fledgling Donetsk People's Republic, its leaders have declared attempts to hold the presidential vote, scheduled for May 25, on their territory illegal.

It may be impossible to hold the vote in Ukraine's rebel regions. Read more here.

Now, with the vote for Ukraine's next president only a few days away, pro-Russia rebels operating in country's east are stepping up a campaign of violence and intimidation that they hope will stop the vote from going ahead.

Militia groups have kidnapped numerous election officials and local politicians across the eastern regions in the weeks building up to the vote. Many have been released after a few hours with a stern warning to keep their heads down — but not all.

On May 8 armed men showed up at the home of 44-year-old Alexey Deniko, a member of Ukrainian nationalist party Svoboda. The rebels smashed up furniture in the house before leading the activist away at gunpoint.

Deniko was later dumped, after being badly beaten and with a serious bullet wound. During a transfer to a hospital in nearby Dnipropetrovsk he was kidnapped after the ambulance transporting him was ambushed as it tried to leave the city.

"He's still under the detention of separatists as far as I know, I think he is being held in Gorlovka," his wife Marina told VICE News by telephone from a secret location.

Marina, who fled her native city after the attack on her husband, says Alexey has been allowed intermittent contact with her by the rebels, but she is "exceptionally worried" about his welfare.

Other ominous threats have been made against those backing Kiev. On Tuesday, officials in Nikolaev called in police and security services after a stinking package was delivered to the office of the city's acting mayor, Yuri Granaturova, from an unknown sender in Odessa. Once unwrapped, police found a rotting dog's head with a fake grenade stuffed in its mouth.

Police discuss the incident at Granaturova's office.

On the other side of the fence, Ukraine's interim President Oleksandr Turchynov today insisted that Ukrainian forces would push their line forward in the area surrounding Sloviansk, the rebels' heartland.

Visiting the Izyum frontline of a Kiev-backed anti-terrorism operation in eastern Ukraine, Turchynov told VICE News that "the special services are ready for the final phase of the anti-terrorist operation… we will be able clear the terrorists out of the Luhansk and Donetsk regions."

The offensive has claimed dozens of lives on both sides since it entered an "active" phase earlier this month.

"The elections [in rebel-controlled regions] will be held in normal conditions," added Turchynov, who pledged to "address" the problems facing the organization of the vote.

Despite the president's thinly veiled threats, the armed militia operating in the region shows no signs of backing down.

On the same day that district 44 was hit by the rebel gunmen at least 11 other election commissions up and down eastern Ukraine were also reportedly stormed, and election materials stolen. The targeted commissions stretched from Artemivsk, 27 miles from the rebels' heartland in Sloviansk, to Mariupol, an industrial port city in the southeast where earlier this month at least seven were killed by the Ukrainian army during a bungled anti-terror sting.

Ukraine is now detaining journalists too. Read more here.

Not only are the militia groups' seizures of election materials rendering the vote in eastern Ukraine logistically nearly impossible, they are also terrifying potential voters and supervising officials alike.

Oleksandar Kliu, an analyst at a local election monitoring non-governmental organization, told VICE News that as it currently stands at least 30 percent of the polling stations will be unable to open because of staff shortages. This figure, he says, takes account of the legislative changes made by the government in Kiev on Tuesday, which lowered the number of election supervisors legally required for a voting point to open from 13 to eight.

But the number of unopened stations is likely to grow higher as the date edges closer.

"We're working underground now," Aleksandr, a representative of presidential candidate Sergey Tihipko and the head of a Donetsk district election commission, told VICE News. "I don't even tell people about my work anymore."

Aleksandr, who in his regular life runs a car wash company, says that election campaigns have been put on ice this year. "Normally we distribute leaflets and other campaign materials," he told VICE News. "But now it is too dangerous to do such political actions."

"We are all afraid," he added, anxiously glancing at the live footage from the CCTV cameras, which surround his car wash. "I am very worried about my business, they could burn it down."

Follow Harriet Salem on Twitter: @HarrietSalem