"We see armed men who oppose the Kiev government acting completely outside the law in the east," said Yulia Gorbunova, a researcher covering Ukraine for Human Rights Watch. "They have absolutely no legal status to be seizing or 'arresting' people, as they call it."
The rights group is also voicing concerns over the Ukrainian authorities' prosecution of separatists for speaking out in favour of secession, as opposed to engaging in acts such as the seizing of buildings. The group wants Kiev to "be careful" as it relaunches its counterterrorism operation in the east, Ms Gorbunova said.
Among Horlivka's modest cohort of pro-unity leaders, the most pressing task is to uphold the rule of law and maintain peaceful political discourse in a region where the will of the mob increasingly prevails.
The town hall of Horlivka, population 270,000, was seized briefly on April 14 during a demonstration of about 1,000 pro-Russia separatists who named a "people's mayor" and raised the Donbass flag. The protesters left two days later and the city's elected mayor, Evgenyi Gleb, has since regained control.
Mr Rybak's death sent a chill through the city's pro-unity officials, who on Wednesday remembered the slain man as a passionate, at times impulsive figure who took part in many demonstrations and was unafraid of voicing his views. "He was a real 'oppositionist'," Mr Zhuk said. "He openly spoke out against power."
Read MoreAs Ukraine ends truce, is Russian invasion next?
City councillors are split between those who support Kiev and those who favour a looser federation or closer alliance with Russia. They have left the Donbass flag untouched above the building since Mr Rybak's disappearance but business inside – unlike some other government buildings in the east – continues as usual, unimpeded by pro-Russia occupiers. Some councillors wear Ukrainian flag buttons on their lapels.
Townspeople have in some cases stepped in to defuse tensions in place of a police force they describe as demoralised, dysfunctional and split by the political issues of the day. In one recent case, when a member of a pro-Russian "self-defence" militia broke a woman's car window at a roadblock, it was ordinary citizens who stepped in to turn the man in to the police.
Amateur footage of Mr Rybak's abduction that surfaced this week show police standing by as the politician was shoved, pulled and insulted by the crowd before being hustled away by his assailants.
Oleg Gubanov, an independent councillor, said the city was still in control of its citizens. He said he had not seen a single Russian in Horlivka but "criminals" were making their way into the city.
"My personal opinion is that there is a third force that is trying to destabilise the situation," Mr Gubanov said. "What we need to do is find common sense."
Follow us on Twitter: @CNBCWorld