The men, dressed in irregular fatigues and with balaclavas pulled over their heads, fingered their Kalashnikovs nervously and jumped at every unusual sound. Eager to aid their country's military struggle, the so-called Donbas volunteer battalion was ready to fight, but appeared to be short on training.

The battalion commander, Semyon Semenchenko, a 40-year-old from Donetsk with a degree in film-making, insisted that he and all his men had combat experience, from the Ukrainian or Soviet armies. They are all volunteers, receiving zero salary from either the state or oligarchs, he said, claiming they live off their own savings and donations from patriotic Ukrainians, who transfer them money after reading about them on social media.

"Our state needs defending, and we decided that if the army could not do it, we should do it ourselves," said Semenchenko, during a meeting with the Guardian outside the town of Mariupol, where his men were based and offering support to regular units of the Ukrainian army in their fight against armed separatists in the region.

With military operations inside Ukraine's borders an unappealing prospect for many of the country's professional soldiers, irregular units are springing up as Kiev struggles to wrest back control of Donetsk and Luhansk regions from the grip of pro-Russia fighters. They have been given semi-legitimacy by the Ukrainian authorities, grateful for any help they can get in their fight in the east.

"It is hard to trust the army and the national guard," said Semenchenko. "There are cases when they have just given up their weapons and fled. I don't understand it at all, how can you give an oath to a country and then not stick to it?"

Volunteers are recruited from western Ukraine and Kiev, and more quietly, within the east itself. A self-published newspaper in Donetsk gives the phone number where "Ukrainian patriots" can sign up for the volunteer battalions; its editor has gone into hiding to avoid being kidnapped by the separatist fighters. Volunteers undergo training in neighbouring Dnepropetrovsk region, and their battalions can be brought under the command of the interior ministry, allowing them to operate legally. Nevertheless, the training period can be as little as 50 hours, before the volunteers are put into real combat situations.

Arming troops with almost no real training and sending them into extremely sensitive situations where they may be shot at with weapons from within crowds, largely made up of angry but unarmed civilians, sounds like a recipe for disaster.

Members of a self-defence battalion take part in a training exercise. Photograph: Gleb Garanich/Reuters/Reuters

Indeed, it has resulted in bloodshed on a number of occasions so far, most notably in Mariupol last Friday, when at least eight people died when the national guard entered the city to clear the police station of separatist fighters. On their retreat, troops fired at civilians, almost all of whom were unarmed.

These incidents, already awful enough, are often amplified and distorted by Russian media, leading to even more anger among the crowds in what is becoming a downward spiral of hatred and violence.

Kiev's "anti-terrorism operation" in the east of the country involves units of the army, the police, special forces and the national guard, which is partly made up of volunteers drawn from those who participated in the Maidan protests in Kiev.

Andriy Parubiy, head of Ukraine's national security and defence council, told the Guardian that these were all coordinated from a single anti-terrorism command centre, but numerous sources on the ground attest to the fact that coordination is poor, and there are major concerns over how ready the volunteer brigades are for combat.

In addition to the difficulties of coordinating such a diverse range of paramilitary groups, there has also been concern at the extreme nationalist element among those fighting. The frequent Russian claim that the Ukrainian government itself is fascist is untrue, but there are certainly far-right elements involved in the fight in the east.

Parubiy himself has an extremely dubious past, having set up the neo-fascist Social National party of Ukraine together with the current leader of far-right Svoboda, Oleh Tyahnybok, in the early 1990s. While there has been little evidence that the militias have been motivated by any kind of far-right ideology when fighting in east Ukraine, there is no doubt that radicals have been the people most willing to fight, and this has led to a number of situations which appear to be well beyond the bounds of normal military behaviour.

In one incident, the radical politician Oleh Liashko was shown in footage that emerged last week humiliating captured insurgent and self-proclaimed defence minister of the "Donetsk People's Republic", Igor Kakidzyanov.

A video of the interrogation, where Kakidzyanov was shown in his underwear with his hands bound, circulated widely on social media and was promoted by Liashko himself.

"This whole situation is completely out of control," said Anna Neistat, an associate director at Human Rights Watch, who is currently in eastern Ukraine.

Five days after the incident, Parubiy told the Guardian that he had not even watched the footage, which also appeared to show Liashko ordering around armed men, and there had been no formal condemnation from the government.

Posters promoting Liashko's presidential campaign read: "Death to the Occupiers!" and are widely displayed all over cities across western Ukraine.

Speaking to the Guardian by telephone, Liashko said he conducted the interrogation because he wanted to find out what the motivations and ideas of Kakizdyanov were. He said he did not think it inappropriate that he was allowed to carry out the interrogation, nor that the questioning took place with Kakidzyanov stripped to his underwear.

"I had before me a terrorist and I wanted to understand how he thinks; what his goals, motivations and ideals were," said Liashko. "It turned out he was in close contact with Russian intelligence; it just proves that the people we are dealing with are Russian agents."

Liashko is currently in the process of setting up his own volunteer battalion, which he hopes will become another addition to the motley selection of forces currently fighting for Kiev in the east.

"For 23 years nobody has paid any attention to our army, and now when we need to fight for the borders of our country today, we can't," he said.

"We need a people's war, like in the second world war when people rose up to fight fascism, that's what we need to do now."

Pro-Ukrainian self defence unit performs weapons practice at their training ground outside Donetsk, Ukraine. Photograph: Efrem Lukatsky/Associated Press

Liashko said that he would be the "commissar" of the battalion but that it would take military orders from the army or the interior ministry. So far, he said, over 3,000 people had applied to join, of which around 400 had been selected. The criteria were that they should be physically fit, have combat experience, and undergo a background check to ensure they were not working for foreign intelligence agencies.

"We are fighting against terrorists and we will work according to the principle: if they don't surrender, they should be destroyed," said Liashko. "Russian mercenaries are trying to turn Donbas into a second Chechnya, and we cannot allow it."

Russian media reported earlier in the week that Liashko had been captured by rebels, but he later emerged unscathed, announcing his security by posting a photograph of himself, his mother and a large white cat on his blog. He told the Guardian that four pro-Russia separatists had been killed and three captured during the attempt to take him hostage, but gave no further details.

With the new militias often fighting in unmarked uniforms, it has sometimes been difficult even to identify who they are. In one incident during Sunday's unrecognised referendums on independence, a group of militiamen arrived in the town of Krasnoarmeisk, supposedly to stop people from voting.

They said they were from the "Dnepr" volunteer battalion, a similar outfit to the Donbas battalion, made up of volunteers and trained in neighbouring Dnepropetrovsk region, funded by the local governor-oligarch, Ihor Kolomoysky.

There was shouting and aggression from the crowd about the men who had disrupted the voting. At one point, several people lunged towards them, unarmed, and the men shot into the air. The volley of bullets did nothing to placate the crowd, and the men kept shooting, a look of panic on their faces. The incident ended with two civilians dead, and later the Dnepr battalion claimed its forces had never been there.

Exactly who the men were remains unclear, and the Ukrainian government has said it will investigate. Photographs from the event appear to show one of the deputy leaders of Right Sector involved in the incident.

The Right Sector is a loose grouping of ultra-radical elements that led confrontations with riot police in Kiev, throwing molotov cocktails and wielding baseball bats. The group's influence has been consistently distorted by its own boasts and Russian state media exaggerations, but it is clear that some of its members are fighting in the east, presumably within volunteer battalions.

It is Right Sector that is most often mentioned as the fascist component of the new government. Although its leader has met with the Israeli ambassador to Ukraine and insisted that the group does not adhere to racial ideology, it is clear that it contains some extremely far-right elements.

One 18-year old Right Sector member, who gave his nickname as "White", claimed that he was involved in fighting in the east and had been wounded outside the insurgent-controlled town of Slavyansk.

"People are terrified of Right Sector and think that we will kill children, but we don't make a big show of it [in the east] and we wear different uniforms without recognisable insignia," he said while patrolling in central Kiev with a gas mask and a rubber truncheon.

In addition to the huge number of different groups fighting on the Ukrainian side, there is also a ragtag assortment of people fighting for the separatists – a mixture of Cossack militias and others from Russia who may have links with Russian intelligence, people representing local business and criminal interests, and ideologically motivated locals who genuinely believe in the cause.

Insiders say there are already extreme tensions between the various armed groups that make up the forces of the self-proclaimed Donetsk People's Republic, and there have been exchanges of fire between different pro-separatist groups on more than one occasion.

"I hope it does not progress further, but there is a tendency of moving towards the scenario we saw in the Yugoslav wars," says Ihor Todorov, a professor at Donetsk National University. "We can end up with different field commanders, who are fighting against everyone; not for a particular side but just for their own ends."

For now, all-out infighting between groups ostensibly on the same side has been prevented by a stronger hatred for the enemy, as both the pro-Kiev and pro-separation forces have cultivated a hatred for their opponents.

On the pro-Russia side, the gunmen regularly speak of the Ukrainian army as "fascists". Rumours that Ukrainians are forced to go through psychological training that allows them to kill unarmed women and children with no remorse are widespread.

On the Ukrainian side, too, there is little sympathy for the views or goals of those they are fighting against.

Semenchenko, of the Donbas volunteer battalion, was uncompromising about civilian casualties, claiming that many of the unarmed people in the crowds were paid to be there as cover for armed attackers, and referred to them as "pigs". It was the "terrorists" who were responsible for genuinely unarmed protesters being inadvertently shot by pro-Kiev forces, such as in Mariupol, he said.

It is the sort of language that precedes civil wars, and talking of Russian anger that the bloodshed in Mariupol had come on Victory Day, Semenchenko he did not believe that the pro-Russians had anything to celebrate.

"My grandfather also fought in the second world war. I think these people are the grandchildren of traitors, secret policemen and collaborators, as real heroes could not produce such grandchildren."