This eastern Ukrainian city took another step toward mob rule Thursday as pro-Russian separatists stormed the state prosecutor’s office and forced dozens of riot police deployed to guard the building into a humiliating surrender.

The attackers, who threw stones and wielded sticks, were backed by a crowd of at least 1,000 men and women of all ages. They chanted “fascists” and “traitors” at the riot police and waved Russian flags as well as those of the self-styled Donetsk People’s Republic.

Ukraine’s acting president admitted this week that the police force in the east was “helpless” to protect citizens and that some of its members were colluding with pro-Russian groups.

On Thursday, citing Russian interference in his country’s affairs and “threats of encroachment” from tens of thousands of Russian troops massed just across the border, Oleksandr Turchynov signed a decree reintroducing compulsory military service for men ages 18 to 25.

In Moscow, Russian President Vladimir Putin made what appears to be a politically impossible demand: that the Ukrainian government completely withdraw its troops from the southeastern part of the country. He made the comments in a telephone conversation with German Chancellor Angela Merkel, who had called him about the deteriorating security situation in eastern Ukraine.

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 government in Kiev stormed the general prosecutor’s office in the eastern regional capital of Donetsk.

‘Put them on their knees’

Arriving at the office, the mob ripped up stone walls at a nearby park to make rocks to use as projectiles. The protesters, some wearing black balaclavas, smashed windows and quickly forced the riot police on the steps of the building to retreat inside, seizing some of their shields as they did so.

Gunshots and small detonations rang out, and clouds of tear gas enveloped the building as the struggle continued inside. At one point, a Ukrainian armored personnel carrier approached the building in an attempt to relieve the siege but retreated when the mob turned in its direction.

Later, a group of policemen cowered outside the building, crouched on the ground with their shields held over their heads, before finally surrendering. Stripped of their riot gear, they were led away through the crowd, their heads bowed as some separatists struck and spat at them.

A woman carrying a small Russian flag on a pole shouted “parasites” as one policeman was put into an ambulance, while others in the crowd bayed for blood.

“They beat our guys! We should beat them, too. We should put them on their knees,” a man and a woman yelled. But a pro-Russian activist in a riot helmet defended the police. “They are just officers on duty, doing their job,” he shouted back.

A song called “The Russians Are Coming,” as well as another popular song written at the time of the Soviet war against Nazi Germany, blasted from the speakers of a car. The vehicle flew a flag carrying an image of World War II-era Soviet leader Joseph Stalin.

Men with close-cropped hair and tattoos stood triumphantly on the steps of the captured building, banging confiscated batons on riot shields in unison as the crowd chanted “bravo.” Some carried pistols. A Ukrainian flag and coat of arms burned in a small bonfire close by, along with a pair of women’s high-heeled shoes.

Inside the building, looters grabbed items ranging from legal files to hard drives.

Regional authorities said 26 people were hurt, including four with gunshot wounds or injuries from rubber bullets. One policeman was hospitalized.

The prosecutor was appointed by the national government in Kiev two months ago. He was investigating corruption charges against pro-Moscow former president Viktor Yanukovych, who was ousted in February after street protests in the capital and fled to Russia.

The Donetsk region of eastern Ukraine was the heartland of Yanukovych’s support, and Thursday’s violence appeared partly orchestrated to prevent any investigation into the ­sources of his wealth here, as well as to attack a symbol of Ukrainian rule.

May Day demonstration

The demonstration, on International Workers’ Day, also known as May Day, began under a huge statue of another former Soviet ruler, Vladimir Lenin, with some flying the flag of the Soviet Union.

“For 23 years, I wanted to be in the Soviet Union,” said a 35-year-old miner who gave his name only as Sergey. Asked why, he said: “I don’t want to live with gays.” Later, Sergey, who came to the demonstration with his wife and son, stood among the crowd with a rock in his hand.

The demonstrators then moved to the city police headquarters, where they forced the police to hang the flag of the Donetsk People’s Republic outside the building.

Demoralized, corrupt and poorly paid, the police in eastern Ukraine have been caught in the middle and have attracted the scorn of both sides.

On Monday, riot police in Donetsk stood by and watched as a pro-Russian mob attacked a peaceful rally in which about 1,000 marchers called for Ukraine to remain united. The city council denied permission for another pro-Ukrainian rally on Tuesday, ostensibly because it represented a “threat to the people” of Donetsk, according to a letter seen by The Washington Post.

On Tuesday, the main police station in the eastern city of Luhansk was also stormed by a separatist mob.

Michael Birnbaum in Moscow and Anna Nemtsova and Alex Ryabchyn in Donetsk contributed to this report.