The monitors, who are linked to the Vienna-based Organisation for Security and Co-operation in Europe, gave assurances they were not being mistreated.
One man who was reported to be Swedish was later released on medical grounds. A spokeswoman for the city’s self-proclaimed mayor, Vyacheslav Ponomarev, said that Major Thomas Johansson was “released on humanitarian grounds as he has a mild form of diabetes”.
The insurgents in Slovyansk have taken a number of people hostage, including journalists and pro-Ukraine activists, as they strengthen their control in the east of the country.
Yesterday, they captured three Ukrainian security service officers, who were shown to Russian journalists bloodied and blindfolded with packing tape.
Also yesterday, a crowd of several hundred pro-Russia activists stormed the television broadcasting centre in Donetsk, the regional capital of eastern Ukraine, to demand that Russian state channels be put back on the air. The Kiev government last month blocked the broadcasts of the Russian channels, which serve as propaganda tools for the Kremlin.
The crowd included several dozen men wearing camouflage fatigues and face masks, the standard uniform of the pro-Russia forces that have seized government buildings in at least ten cities in eastern Ukraine.
Colonel Axel Schneider from Germany, who spoke for the group of military observers detained on Friday, stressed that they were on a diplomatic mission and were not spying for Nato, as the insurgents claim.
Col Schneider said additional proof of this was the participation of the officer from Sweden, which is not a member of Nato.
The observers, who appeared nervous, were in the custody of armed men wearing camouflage fatigues and black balaclavas, who escorted them into the Slovyansk city hall for the news conference and led them away afterwards. Col Schneider, however, said they were being treated as well as possible under the circumstances.
“The mayor of this city granted us his protection and he regarded us as his guests,” Col Schneider said. “I can tell you that the word of the mayor is a word of honour. We have not been touched.”
Col Schneider said he had no information about when they would be released and that this was a matter for diplomats of their countries. In addition to the free Swede, the group includes three German officers and a civilian interpreter, and officers from Poland, Denmark and the Czech Republic.
The German colonel said he understood that Mr Ponomarev could use the observers as a bargaining chip.
“Our presence here in Slovyansk is for sure a political instrument for the decision makers here in the region and the possibility to use it for negotiations,” Col Schneider said.
Mr Ponomarev said on Saturday that the European observers could be released in exchange for jailed pro-Russia activists.
Yesterday the insurgents captured three Ukrainian security service officers, who were shown to journalists in the security service headquarters in Slovyansk.
Stripped of their trousers and shoes, they sat with heads bowed.
Igor Strelkov, who has been identified as the commander of the armed insurgents, said the three Ukrainian officers were on a mission to seize leaders of the pro-Russia force when they were captured.
Ukraine’s security service confirmed that its officers had been seized by armed men.