Drunken mobs roamed the otherwise deserted center of this eastern port city Saturday a day after clashes between pro-Russian militants and Ukrainian security forces left at least seven people dead and the region as a whole continued to seethe on the eve of referendum to secede.

A deadly firefight Friday broke the calm of a patriotic holiday and heightened tensions ahead of Sunday’s hastily organized plebiscite to break with the interim government in Kiev. Late Friday, a group of unidentified assailants attacked and briefly detained several Red Cross volunteers, including a French citizen, at an office in Donetsk.

Iryna Tsaryuk, an employee of the Ukrainian office of the International Committee of the Red Cross in Kiev, told the Interfax news service Saturday that the seven employees who had been taken captive were released about 2 a.m. and that one had been hospitalized after a severe beating. Roman Lyagin, the head of the rebel election commission, denied that rebels were involved in the incident.

In another sign of the collapsing morale of Ukraine’s security forces, the national guard evacuated its barracks in Mariupol on Saturday morning, apparently hastily. In the city center, meanwhile, activists set fire to a broken-down infantry fighting vehicle that the army had abandoned the previous day, causing a string of explosions and an upsurge of panic, according to witnesses.

Acting President Oleksandr Turchynov again called on rebellious citizens in the east to work out their differences through negotiations, saying the national government was willing to discuss more local control and to guarantee minority rights for ethnic Russians.

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“We can hear Donbas, and we are ready to sit down at the negotiating table,” the presidential news service quoted Turchynov as saying, according to Interfax. Donbas refers to the Donets Basin, an industrial region that spans eastern Ukraine and southwestern Russia.

While Russia is expected to advocate a “yes” vote in Sunday’s referendum, the government in Kiev has been joined by the United States and Western Europe in denouncing the initiative.

On Saturday, the U.S. State Department called the referendum illegal and “an attempt to create further division and disorder” and said the United States would not recognize the results if the vote proceeds. German Chancellor Angela Merkel also called the referendum “illegal,” while French President Francois Hollande said it carries “no weight,” the Associated Press reported.

Friday’s holiday began hopefully enough as most of the country marked Victory Day with few signs of the violent clashes that many Ukrainians had feared after weeks of escalating conflict. But soon, Ukraine was dragged back into a fraternal divide that has verged on civil war and has put the region on edge.

Security forces rolled armored personnel carriers into this crucial port city to take back the police department from pro-Russian militants who had stormed the building in what officials said appeared to be an attempt to seize weapons. At least seven people were killed, they said.

Unlike in other parts of eastern Ukraine, the security forces in Mariupol seemed determined not to cede control of the city without a fight. The heavily armed forces arrived about 10 a.m. and fought a fierce two-hour battle to retake the building.

By afternoon, the police station was a smoldering ruin. A plume of smoke rose into the sky where the roof had collapsed. The walls were charred and pockmarked with bullet holes, and there were at least three large cavities apparently caused by heavy weapons.

Two bodies lay in the street as an angry crowd milled about. The regional government said its health department had reported that seven people were killed and 39 wounded. Ukrainian Interior Minister Arsen Avakov said the dead included one Ukrainian policeman and that four separatist fighters had been captured.

Mariupol, a city of half a million people, is the main port of Ukraine’s eastern Donetsk region and a key prize in the struggle between pro-Russian forces and the Kiev government. The separatists’ influence appears much weaker in this city than elsewhere in the region, and it is far from clear that they will be able to stage their promised referendum here.

The fighting broke out at about the same time that Russian President Vladimir Putin landed a few hundred miles to the south in newly annexed Crimea to join the festivities there marking the Soviet victory in World War II. Joyous crowds greeted him as a hero.

Ukrainians and others, meanwhile, tried to guess Putin’s next move. This week, with the United States and its allies seeking both a diplomatic solution to the crisis and warning of more sanctions against Russia, Putin appeared to calm tensions by calling on pro-Russian activists to postpone Sunday’s referendum. The separatists, however, said the vote would go forward as planned.

Marking Victory Day

Before the fighting, the morning’s Victory Day festivities offered a reprieve from the clashes between the separatists and supporters of a united Ukraine. Even in tense areas, the celebrations went off with only minor incidents. Most Ukrainians observed the holiday with solemn graveside tributes to those who gave their lives fighting Nazi Germany for the Soviet Union, but few of the grand parades that usually accompany the occasion were held, because of security concerns.

In Donetsk, one of the strongholds of the separatist movement, only a scuffle or two broke out as about 2,000 people gathered at a Soviet war memorial for the city’s official celebration of victory in World War II, which many Russians and Ukrainians know as “the Great Patriotic War.”

Elderly veterans marched to the event, many in uniform and wearing medals. Donetsk’s elected mayor gave a speech commemorating the people’s sacrifices as flags fluttered in salute to the Soviet army. Ukrainian flags were absent, but the Ukrainian national anthem was played twice. No one booed the anthem, but almost nobody sang along, either.

As the ceremony came to an end, a man in a camouflage uniform briefly scuffled with a lone policeman who had apparently sung the anthem. Another man in camouflage seized the microphone to complain about the anthem.

“If we continue to obey the fascist junta, our children will become fascists and nationalists,” he said. He then shouted “Donbas,” and many in the crowd clapped.

In Odessa, the Black Sea port city where more than 40 people died in violent clashes a week ago, a few hundred people gathered in Shevchenko Park to lay flowers at a memorial. Some carried banners blaming authorities for the May 2 fighting.

“It is very important for us to declare that we would never stop our struggle for changes in Odessa, [and] never forgive Ukrainian nationalists for attacking and burning our people,” said Vera Butuk, one of the survivors of a fire in the city’s Trades Union House that killed dozens of separatists.

When the official ceremonies concluded, about 150 pro-Russian activists marched to Kulikovo Square, shouting slogans denouncing Ukraine’s interim government. “No to fascism!” they cried. “Fascism will not succeed!”

Proposal by Kiev

In Kiev, church bells pealed about 10 a.m. around the city, and when they fell silent, music wafted from encampments in Independence Square, where a protest movement began last winter against Ukraine’s pro-Russian president, Viktor Yanukovych, who was ousted in February. Bagpipes wailed and drums beat a marching cadence for a sparse crowd that listened to speakers and performers in a low-key observance of the holiday.

At the Park of Eternal Glory in Kiev, Turchynov, the acting president, joined former heads of state to lay a wreath at a memorial to those who died in World War II. Security was heavy and people were screened at metal detectors. Ukrainian officials canceled the city’s parade and urged civic organizations to refrain from staging large events.

On the eve of the holiday, Turchynov and acting Prime Minister Arseniy Yatsenyuk issued a joint statement urging pro-Russian militants to lay down their arms immediately in exchange for amnesty. In the name of Ukrainian unity, they said, the interim government would use “all legal means” to restore order in restive areas.

They also pledged to conduct talks with all political parties and regional representatives on subjects such as local autonomy and minority rights. The Ukrainian leaders said those talks would be held with the assistance of the Organization for Security and Cooperation in Europe.

“The purpose of this dialogue is to develop a national consensus on key issues for the Ukrainian society,” they said. “We invite all political actors and the public to actively participate in this initiative.”

Kunkle reported from Kiev. Michael Birnbaum in Moscow, Alex Ryabchyn in Donetsk, Anna Nemtsova in Odessa and William Branigin in Washington contributed to this report.