Russian state media said Crimea voted overwhelmingly to break with Ukraine and join Russia on Sunday, and Ukraine's Prime Minister Arseniy Yatsenyuk accused Moscow of pouring forces into the peninsula and said of separatist leaders that "the ground will burn under their feet."
According to the head of Crimea's referendum commission, Mikhail Malyshev, 97 percent of ballots favored an alliance with Russia. The announcement, which came just hours after polls closed, also said turnout was 83 percent — a surprisingly high number, given the expectation that many who were opposed would boycott the vote.
The Sevastopol City Council on Monday voted to ratify the referendum results, declared independence and asked to become part of Russia. It also recognized the Crimean Parliament's declaration of independence. The votes were unanimous, but some council members were not allowed into the chamber, where nine seats remained empty.
Yatsenyuk vowed to bring separatist forces to justice even before the Sunday vote on whether to demand greater autonomy from Ukraine or to split off and seek to join Russia was complete. The referendum was condemned as illegal by the United States and European countries.
Washington was quick to voice its rejection of the Crimea vote. "The United States has steadfastly supported the independence, sovereignty and territorial integrity of Ukraine since it declared its independence in 1991, and we reject the 'referendum' that took place today in the Crimean region of Ukraine," a White House statement said.
"This referendum is contrary to Ukraine's constitution, and the international community will not recognize the results of a poll administered under threats of violence and intimidation from a Russian military intervention that violates international law," it continued.
President Barack Obama reiterated this stance in a call with Russian President Vladimir Putin, saying the referendum was "illegitimate" and "occured under duress."
According to a Kremlin statement, Putin told Obama the referendum was legitimate, and he expressed concern about the Ukrainian government's failure to stamp out what Moscow has said is persistent violence against ethnic Russians in the country.
The Russian Foreign Ministry called Monday for the creation of a working group involving international partners to "resolve the situation in Ukraine," according to a statement, while recognizing the right of Crimea to self-determination "in accordance with results the referendum."
"As has been repeatedly explained to the Russian side, the current situation in Ukraine was created not by us but is the result of a profound crisis of the Ukrainian state, which led to polarization and the sharp exacerbation of antagonism between different parts of the country," the ministry said in a statement.
At the United Nations last week, Russia vetoed a Security Council resolution declaring the referendum illegal; China abstained — a sign of Moscow's isolation on the issue.
The Sunday vote took place several weeks after Russian-led forces took control of Crimea, a strategic and predominantly ethnic-Russian region of Ukraine. Many of its residents say they fear they will be oppressed under the Ukrainian government that took over when pro-Russia President Viktor Yanukovich was ousted last month.
Still, the mood in Crimea appeared peaceful and even festive Sunday, and voters were widely expected to back union with Russia.
On Sunday in Sevastopol, the Crimean city where the Russian Black Sea Fleet is based, under a lease agreement with Ukraine, enthusiasm for the referendum was high, with voters lining up outside polling stations before they opened. More than 70 people surged into a polling station in the first 15 minutes of voting.
"Today is a holiday," said one of them, 66-year-old Vera Sverkunova. Asked how she voted, she broke into a patriotic war song, singing, "I want to go home to Russia. It's been so long since I've seen my mama."
But Crimea's Tatar Muslim minority opposes incorporation into Russia.
The referendum "is a clown show, a circus," a leader of the community, Refat Chubarov, said on Crimea's Tatar television station on Sunday. "This is a tragedy, an illegitimate government, with armed forces from another country."
Blue and yellow Ukrainian flags were nowhere to seen on the streets of Crimea's regional capital, Simferopol; red, white and blue Russian and Crimean flags fluttered on sidewalks, city buildings and many cars.
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 that they said was stage-managed by Moscow. Some said they were afraid of the potential for ethnic cleansing in the coming weeks, as happened in parts of the former Soviet republic of Georgia.
"We're just not going to play these separatist games," said Yevgen Sukhodolsky, a 41-year-old prosecutor from Saki, a town outside Simferopol. "[Russian President Vladimir] Putin is the fascist. The Russian government is fascist."
On Sunday pro-Russian hackers claimed responsibility for bringing down several public NATO websites — attacks that appeared to be linked to the tensions over Crimea.
NATO spokeswoman Oana Lungescu said on Twitter that the cyberattacks, which began on Saturday evening, continued on Sunday, although most services have been restored.
A group calling itself Cyber Berkut said the attacks were carried out by patriotic Ukrainians angry over what they saw as NATO interference in their country. "Berkut" is a reference to the feared and now disbanded riot squads used by Yanukovich's government.
The claim, made at www.cyber-berkut.org, could not be independently verified.
Al Jazeera and wire services