DONETSK, Ukraine—The Kiev government appointed a billionaire metals tycoon as governor of this industrial town some 100 miles from the Russian border Sunday, but Pavel Gubaryev says that job is already his.

Standing in his apartment in a Soviet-era block, the owner of an advertising firm says he was elected at a rally of pro-Russian protesters on Saturday.

"People had their say, and I'm ready to take responsibility," he said in an interview.

In any other circumstances, no one would take Mr. Gubaryev seriously, even though he does boast one key accoutrement of power: a "security service" of a dozen men of various shapes and sizes in jeans and jogging pants.

But in today's Ukraine, Mr. Gubaryev might just have a chance. Some 250 miles away in Crimea, a similarly low-profile local, Sergei Aksyonov, was named regional leader after anti-Kiev protests. He promptly rejected the authority of the new government in the capital, swore loyalty to the ousted Russian-backed president, and appealed to Moscow to send troops.

It's not clear whether Mr. Gubaryev, 30 years old, could pull that off here, but his sudden rise to prominence is an example of how the revolution in Kiev has created political vacuums across Ukraine's less-revolutionary, and more pro-Russian, east and south, as even ousted President Viktor Yanukovych's former supporters have accepted the new government.

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Hundreds of Russian troops surrounded a Ukranian military base in Crimean town of Perevalnoe early Sunday morning. Photo: Getty Images

Supporters of Ukraine's new government and pro-Russian demonstrators rallied separately in the eastern Ukrainian city of Donetsk, while thousands gathered in Kiev calling for unity in the country. Photo: Getty Images

Protests against Kiev authorities spread over the weekend in eastern Ukraine, in such cities as Kharkiv, the country's second largest, and the industrial town of Mariupol. Some eastern cities also saw rallies in favor of national unity Sunday.

In Donetsk, the stakes are particularly high as it's at the heart of this industrial region and is the former president's hometown.

Like many here, Mr. Gubaryev has little time for former Mr. Yanukovych, who fled Ukraine for Russia last week, citing a threat to his life. He was later voted out by Parliament.

He also criticizes Mr. Yanukovych's Party of Regions, the main political force here, for its swift acquiescence to the new government, which popular Russian television channels here portray as anti-Russian "fascists." The Kremlin says it could step in to defend the Russian-speaking population, claiming they are under threat of attack from nationalists urged on by the new government.

There have been few independently verified reports of such attacks, but it jibes with people's fears here.

Many say they feel defenseless, especially after Parliament cancelled a language law that allowed Russian as an official regional language.

Amid a power vacuum, Mr. Gutsaryev has taken up that message.

In a fiery speech in the local legislature on Friday, he accused the Party of Regions of supporting "Nazis" in the new government, who he said took power in a coup. He presented an ultimatum: to acknowledge Parliament and the government as illegitimate. Otherwise, his People's Guard of Donbas, named after the region where this town is located, would take matters into their own hands.

"Ready or not, here we come," he said as he ended his speech.

The next day, he forced his way onto the stage at a pro-Russian rally on Donetsk's main square. In front of around 7,000 people, some waving Russian flags, Mr. Gubaryev repeated his ultimatum, before declaring himself governor amid cheering from some sections of the crowd, which Mr. Gubaryev then led to a regional government building. Some replaced Ukraine's flag with Russia's tricolor.

Local officials and witnesses say several dozen Russian citizens, apparently bused across the border, were the most active among the protesters. Many of those who spoke had Russian pronunciation, witnesses said. The governor's office said dozens of "Russian tourists" had been in the crowd.

Moving to head off protests, the city council held an emergency meeting and called for a referendum on the region's status and to cement Russian as an official language in the city.

On the other hand, the city's most powerful tycoon, Rinat Akhmetov, gave firm backing Sunday to the central authorities and the country's unity. And the new governor named by Kiev, Serhiy Taruta, also called on all Ukrainians to "unite to keep our country whole."

But Mr. Gubaryev says he has strong support. He calls himself the "commander" of 8,000 registered members who came together in the last three days via social networks.

That claim couldn't be verified and doesn't seem to agree with his humble surroundings. He gave an interview standing near a baby's crib and a cat's scratch post in an apartment he rents, displaying a bandage on his hand that he said was injured in scuffles Saturday.

"I'm not chasing power," he said.

But Mr. Gubaryev said his new prominence had earned powerful backers in Moscow. He said he'd taken a call Sunday morning from Sergei Glazyev, a top Kremlin adviser, offering support. Mr. Glazyev couldn't immediately be reached for comment. Russian Communist Party lawmaker Vladimir Bessonov also paid a visit to Mr. Gubaryev.

Mr. Gubaryev says he went to Moscow late last month for meetings with sympathetic organizations.

Sunday, around 1,000 people gathered in front of the regional government building, waving Russian flags and chanting, "Russia! Russia!"

Local officials say a flash point could come Monday, when the legislature meets. Police are demoralized and fearful to act, said senior city councilor Sergei Bogachyov.

Mr. Gubaryev says he doesn't want bloodshed, but that many people in Donetsk have weapons, and may be tempted to repeat the events in Crimea, where armed men stormed the legislature, and a newly chosen leader then asked for assistance from Russia

"I don't rule it out," he said.