Despite the sanctions threat, Russian President Vladimir Putin has vigorously resisted calls to pull back in Crimea. At the United Nations on Saturday, Russia vetoed a Security Council resolution declaring the referendum illegal. China, its ally, abstained, and 13 of the 15 other nations on the council voted in favor -- a signal of Moscow's isolation.
German Chancellor Angela Merkel spoke to Putin by phone Sunday, proposing that an international observer mission in Ukraine be expanded quickly as tensions rise in the east. Her spokesman said she also condemned the Russian seizure of the gas plant.
Russian Foreign Minister Sergey Lavrov and U.S. Secretary of State John Kerry also spoke and agreed to support constitutional reforms in Ukraine that could ease the tensions, the Russian Foreign Ministry said.
Ukraine's Regional Policy Minister Volodymyr Groisman told The Associated Press that the new government was already working on giving towns and regions more autonomy but said there were no plans to turn Ukraine into a federation.
In Donetsk, one of the main cities in eastern Ukraine, pro-Russia demonstrators called Sunday for a referendum similar to the one in Crimea, and some of them stormed the prosecutor-general's office.
In Sevastopol, speakers blared the city anthem up and down the streets, providing a block-party atmosphere. But the military threat was not far away -- a Russian naval warship still blocked the port's outlet to the Black Sea, trapping Ukrainian boats.
At a polling station inside a historic school, tears came to Vladimir Lozovoy, a 75-year-old retired Soviet naval officer, as he talked about his vote.
"I want to cry. I have finally returned to my motherland. It is an incredible feeling. This is the thing I have been waiting for for 23 years," he said.
But Crimea's large Muslim Tatar minority -- whose families had been forcibly removed from their homeland and sent to Central Asia during Soviet times -- remained defiant.
The Crimea referendum "is a clown show, a circus," Tatar activist Refat Chubarov said on Crimea's Tatar television station. "This is a tragedy, an illegitimate government with armed forces from another country."
The fate of Ukrainian soldiers trapped in their Crimean bases by pro-Russian forces was still uncertain. Crimea's pro-Russian authorities have said that if those soldiers don't surrender after Sunday's vote, they will be considered "illegal."
"This is our land and we're not going anywhere from this land," Ukraine's acting defense minister, Igor Tenyuk, was quoted as saying Sunday by the Interfax news agency.
But Tenyuk later said an agreement had been reached with Russia that its forces would not block Ukrainian soldiers in Crimea through Friday. It wasn't clear exactly what that meant.
On the streets of Simferopol, blue-and-yellow Ukrainian flags were nowhere to seen, but red, white and blue Russian and Crimean flags fluttered in abundance.
Ethnic Ukrainians interviewed outside the Ukrainian Orthodox cathedral of Vladimir and Olga said they refused to take part in the referendum, calling it an illegal charade stage-managed by Moscow. Some said they were scared of the potential for widespread discrimination and harassment in the coming weeks, similar to what happened in parts of nearby Georgia, another former Soviet republic, after its 2008 war with Russia.
"We're just not going to play these separatist games," said Yevgen Sukhodolsky, a 41-year-old prosecutor from Saki, a town outside Simferopol. "Putin is the fascist. The Russian government is fascist."
Vasyl Ovcharuk, a retired gas pipe layer who worked at Ukraine's Chernobyl nuclear disaster in 1986, predicted dark days ahead for Crimea.
"This will end up in military action, in which peaceful people will suffer. And that means everybody. Shells and bullets are blind," he said.
Dalton Bennett in Sevastopol, Yuras Karmanau in Strilkove and Jim Heintz and Maria Danilova in Kiev contributed to this story.