The Mariupol town hall on Thursday. (Baz Ratner/Reuters)

The pro-Ukraine activists struck before dawn Thursday while the separatists were fast asleep, retaking city hall after nearly two weeks of occupation and notching a small but critical victory in the struggle to keep this country’s eastern half from slipping into Russian hands.

But victory was fleeting: By Friday, the pro-Russia Donetsk People’s Republic had been reinstated in this heavily industrial port city of half a million. Inside city hall, the separatists were busy — restocking supplies of molotov cocktails, brokering deals with the local police and vowing not to yield until they win their freedom from the government in Kiev.

The episode reflects the massive challenge that Ukrainian authorities face as they try to reassert their authority in a region where government buildings remain in separatist hands a week after Russia and the West agreed on a plan to end the occupations.

Rather than accede to demands to leave, the pro-Russia demonstrators have flouted them with increasingly aggressive behavior that tests not only Kiev but also its Western backers.

That behavior reached a new pitch Friday, with separatists detaining a group of European security monitors and branding them “spies.” The development was likely to raise the stakes in Russia’s standoff with the West, which has already left Ukraine dismembered by the loss last month of Crimea.

As Russia has escalated, the Ukrainian government’s loudly advertised “anti-terrorist campaign” — dependent on weak security forces of questionable loyalty — has foundered. With tens of thousands of Russian troops poised to pour into Ukraine at a moment’s notice, Kiev’s ability to influence events across its eastern region appeared to be diminishing by the day.

“The state is weak. That is the main problem,” said Yevgeny, a pro-Ukraine activist in Mariupol who is too concerned about retribution to allow his last name to be printed. “The city authorities don’t know what to do. They don’t want blood. They don’t want conflict. And so the Donetsk People’s Republic is back.”

That dynamic was especially clear Friday at a checkpoint in the eastern city of Slovyansk, where a pro-Russia militia captured a bus containing eight members of an international observer mission as well as five Ukrainian troops and their driver, according to Ukraine’s interior ministry.

The eight observers were part of a German-led military observer mission under the auspices of the Organization for Security and Cooperation in Europe, which is monitoring the conflict in eastern Ukraine and attempting to bring the sides together in talks.

The observer mission had been requested by the Ukrainian government, a European diplomat said Saturday, speaking on condition of anonymity because he was not authorized to give details about the OSCE mission. The observers were planning to be on the ground for a week, but they had arrived in the region not long before they were taken by pro-Russian forces Friday.

A spokesman for the German defense ministry said the mission included three German officers, a German translator and representatives from the Czech Republic, Poland, Denmark and Sweden.

The “people's mayor” of Slovyansk, Vyacheslav Ponomariov, told a Russian media outlet that local militia in his city had taken the observer team and was holding them in the Ukraine Security Service building that his forces have occupied for two weeks.

“This is not a civil mission, because they are soldiers,” the mayor told Russia’s Life News TV. “So they are spies. Also they had a map with all checkpoints, which proves that they are spies.”

The government in Kiev said that the men were “hostages” and that it would negotiate for their release.

The Russian news service Interfax quoted an unnamed separatist official Saturday saying that the detained Ukrainian officers could be swapped for pro-Russian activists who are being held in Kiev. Reached by phone, a spokeswoman for Ponomariov declined to comment on that suggestion, but affirmed that both the captured foreign observers and their Ukrainian counterparts are considered “spies.”

“We are treating them well,” said spokeswoman Stella Khorosheva. “We give them food and water.”

The Russian Foreign Ministry said Saturday that they were “taking all measures possible to resolve the situation.” It added in a statement that the Ukrainian authorities “should have cleared the inspectors’ presence, activity and security in the regions … where a military operation has been launched against the people of their own country.”

Earlier on Friday, separatists blew up a Ukrainian government helicopter at a restricted airfield near the eastern city of Kramatorsk. Ukraine’s defense ministry said it was hit by a rocket-propelled grenade.

By contrast, Mariupol was by all outward appearances calm Friday. The city, hard on the shores of the Sea of Azov, has long been an important center of Ukrainian industry. On Friday, tens of thousands of workers boarded rusted-out streetcars for the journey from their Soviet-era apartment blocks to the hulking factories that ring the town. There was no sign that geopolitics were interfering with the daily commute.

But at the center of the city, separatists dug in behind barricades made of tires and driftwood and readied themselves for the next fight.

On Thursday, the dozens of pro-Russia demonstrators had been caught off guard by the 3:45 a.m. raid of an approximately equal number of Ukraine partisans who had armed themselves with sticks and baseball bats, according to participants and officials.

The police were soon called in, and the defeated pro-Russia demonstrators were escorted out. Ukrainian Interior Minister Arsen Avakov took to Facebook to trumpet the liberation, calling it “good and right” and vowing to continue “the process of normalization.”

But on the ground in Mariupol, the tide was already turning back toward the separatists. As the demonstrators negotiated with city authorities, a crowd of sympathizers gathered around the building.

Mayor Yuri Hotlubey said in a written response to questions that he and other city authorities decided not to move back into city hall because the building had been damaged and would require repair.

Others suggested that the government simply didn’t have the will to keep the motley crew of demonstrators — some in their early teens, many in their 50s or above — from returning.

“The police are supporting us now,” said Evgeniy Pomazan, 50, a sailor who identified himself as the deputy commandant of the occupying force.

On Friday, demonstrators controlled the building’s grounds and four main floors. A small number of police officers stood watch on the top floor and did not interfere as the protesters filled the basement with molotov cocktails.

The demonstrators said they intend to stay until the government holds a referendum on whether to make eastern Ukraine autonomous from Kiev. Critics say the referendum would be a precursor to Russian rule.

Pomazan emphasized that he and his fellow demonstrators had taken excellent care of the government property they had seized, regularly sweeping the floors and laying out mattresses in the hall so as not to disturb the contents of offices.

“After the referendum, this will all be ours,” Pomazan said. “So why should we destroy it?”

Police, meanwhile, asserted that they were hard at work on an investigation into the goings-on at city hall. But nearly two weeks after it was seized, they could not say who had taken it or why, or even whether the separatists’ actions constituted a crime.

“We’ve started the investigation,” said police spokeswoman Yulia Lafazan. “Only the investigation can say whether it’s legal or not.”

Booth reported from Donetsk. Alex Ryabchyn in Mariupol, Michael Birnbaum in Moscow and Stephanie Kirchner in Berlin contributed to this report.