When pro-Russia protesters stormed a police station in Slavyansk, in eastern Ukraine, they seized several hundred firearms. They also took control of the biggest weapons cache in eastern Europe, which lies beneath their feet.

Since March a group of protesters have been guarding the entrance to the Volodarsky salt mine, which holds an underground collection of at least a million firearms ranging from first world war heavy machine guns to Soviet-era Kalashnikovs.

The protesters say they are there to prevent the new government from using the weapons against them, but officials and analysts worry that pro-Russia militias could seize the guns.

"If such a large amount of weapons fell into hands of separatists, it would be a catastrophe," said Alexei Melnik, a defence analyst at the Razumkov centre in Kiev. When Melnik visited the cache in 2002 it held about 3.5m firearms, he said.

The mine is near the village of Paraskoviyevka, 25 miles from Slavyansk, the centre of the uprising by armed pro-Russia militias demanding regional referendums on greater independence from Kiev.

The interior ministry called Slavyansk the "most dangerous place in Ukraine" after locals seized three government buildings and 400 firearms there last week. As many as five people were reported to have died in a midnight shootout in the town on Saturday night.

The protesters, who have blockaded the entrance with a string of tents, identify themselves as self-defence forces of Donbass, the coalmining and metalworking region with strong historical and economical ties to Russia. Many of them support the Donetsk People's Republic, which calls the new Kiev government an illegitimate "fascist junta" and has vowed not to leave occupied government buildings a referendum is held on the region's "sovereignty".

They fear that weapons in the cache will go to arm the national guard announced last month by the acting president, Oleksandr Turchynov, or worse yet to the dreaded Right Sector militias that have become a kind of bogeyman in eastern Ukraine.

"I stand for order, I don't want fascists to come and use our weapons against us," said one of the men, Semyon Khodakovsky, who served with the Berkut riot police during violent clashes at the Euromaidan protests that ousted President Viktor Yanukovich, a Donetsk native. "Everyone knows a fascist movement has seized power in Kiev … they're afraid to stick their nose in here without weapons."

The Soviet government first started gathering arms in the mine in 1957, creating a storage facility with lockers of weapons almost 150 metres beneath the ground, according to one of the protesters manning the blockade. The man, who declined to give his name, said he worked on weapon maintenance at the facility during the 1990s. The weapons included first world war sabres, German rifles and even examples of the horse-drawn machine gun known as a tachanka, he said.

Melnik said the cache also held a large number of Kalashnikovs brought there in the 1990s from all over eastern Europe. The Soviets chose the site because of its defensibility, since the only way in is through a single elevator, he said. In addition, the relatively unchanging climactic conditions are ideal for storing weapons.

The volunteers at the blockade included locals, military veterans and Cossacks, descendants of the historic warrior caste that once guarded the borders of the Russian empire. They said they had set up the blockade and a checkpoint down the road after men arrived in four military trucks and attempted to remove weapons from the cache on 18 March. Hundreds of locals foiled a second attempt soon after, they said.

When the Guardian visited, the protesters were friendly and unarmed, but Khodakovsky said they had firearms in case of an attack. Lieutenant Colonel Eduard Lebedenko, the deputy commander of the storage facility, declined to comment when approached outside the mine on Sunday. Two dozen soldiers guard the cache and another six dozen are stationed in a military installation across the road, according to the volunteers.

The mayor of Slavyansk said last week that armed men from Russia and Crimea were in the city, but the Cossack in charge of guarding the mine in Paraskoviyevka said the blockade included only locals and people from nearby cities. He said no weapons had been removed from the cache recently, despite reports that some military trucks had initially got through.

"We will hold this until we win. No one will get the weapons," said the guard, who identified himself only as Sergei. "I don't want any vermin coming here – not Europe, not America, not Right Sector, not foreign mercenaries.

"I will shoot Yarosh personally," he added, referring to the Right Sector leader and presidential candidate Dmitry Yarosh.

For now, the authorities appear powerless to break through the blockade. The Donetsk governor, Sergei Taruta, recently appointed by Kiev, ordered the regional police head, Konstantin Pozhidayev, to deal with the protesters in March, promising the problem would be solved immediately. But Pozhidayev resigned last weekend after pro-Russia protesters stormed his office, and law enforcement officers have tacitly or directly supported the uprising in large part.

Khodakovsky and other members of the Berkut riot police, who were known as Yanukovich's shock troops, have played a key role in many of the building takeovers in eastern Ukraine after they returned from Kiev. He and other protesters denied Kiev's allegations that they were separatists.

"I want Ukraine to stay whole, but we need to drive out these parasites to save the country," Khodakovsky said. "If there's no other option, then we'll try to join Russia."

Melnik is concerned about the continued security of the cache. "The contingent that guards the weapons can guarantee their security in peacetime against small groups," he said. "But now that we see seizures by big groups with Russian agents, I fear they can't hold out for long."