MOSCOW — Russian President Vladimir Putin demanded Thursday that the Ukrainian government withdraw its troops from the troubled eastern part of the country, where pro-Russian separatists have been gaining ground.

Putin made the demand in a conversation with German Chancellor Angela Merkel, who called the Russian leader Thursday about the deteriorating security situation in eastern Ukraine. She reached out to Putin a day after Ukraine’s acting president said he had lost control of that portion of his country.

Merkel, who will meet with President Obama on Friday in Washington, pressed Putin to help secure the release of seven international observers – four of them German – who have been taken hostage by separatist forces in the eastern Ukrainian city of Slovyansk, her spokesman said Thursday.

Putin told her that “it was imperative today to withdraw all military units from the southeastern regions” of Ukraine, and he called for a “broad national dialogue” about reforms to Ukraine’s constitution, the Kremlin said.

But on the ground in Ukraine, any hope of dialogue appeared to be slipping ever further away, as protesters opposed to the Ukrainian government in Kiev stormed the general prosecutor’s office in the eastern Ukrainian regional capital of Donetsk a day after they had taken over the city council building there.

Crowds of separatists massed in front of Donetsk’s police station Thursday, demanding that all pro-Russian activists be freed across eastern Ukraine. They waved Russian flags – and at least one banner depicting World War II-era Soviet leader Joseph Stalin and the Communist hammer and sickle symbol – and played Soviet patriotic songs. Then they turned their attention to the nearby state prosecutor’s office, where dozens of black-clad riot police with metal shields stood in front of the entrance.

A confrontation quickly ensued, as the riot police attempted to push the crowd away from the entrance with tear gas. The protesters, chanting “fascists,” threw rocks, breaking windows in the office building and demanding that the prosecutor come out. Men in black balaclavas quickly pushed the riot police away from the entrance and forced them to surrender, less than an hour after the pro-Russian protesters had arrived.

If Ukraine’s interim government carries out military operations in the eastern part of the country, it “could lead to disastrous consequences,” the Russian Foreign Ministry warned Thursday.

“We are calling on Kiev, as well as the U.S. and the E.U. indulging it . . . not to commit criminal mistakes and to soberly assess the gravity of possible consequences of using force against the Ukrainian people,” the ministry said in a statement.

In an acknowledgment of his weakness, acting President Oleksandr Turchynov on Wednesday said the Ukrainian government’s goal now was to prevent the agitation from spreading to other areas.