Ukrainian Prime Minister Arseniy Yatsenyuk said there was no reason for Russia to invade Ukraine and warned that "we are on the brink of disaster."
"We believe that our western partners and the entire global community will support the territorial integrity and unity of Ukraine," he said Sunday in Kiev.
NATO held an emergency meeting in Brussels, Britain's foreign minister flew to Kiev to support its new government and the U.S., France and Britain debated the possibility of boycotting the next Group of Eight economic summit, being held in June at Sochi, the host of Russia's successful Winter Olympics.
In Kiev, Moscow and other cities, thousands of protesters took to the streets to either decry the Russian occupation or celebrate Crimea's return to its former ruler.
"Support us, America!" a handful of protesters chanted outside the U.S. Embassy in Kiev. One young girl held up a placard reading: "No Russian aggression!"
"Russia! Russia!" the crowd chanted in Moscow.
Kerry, interviewed Sunday on U.S. television news shows, talked about boycotting the G-8 summit, as well as possible visa bans, asset freezes and trade and investment penalties against Russia. Kerry said all the foreign ministers he had talked to were prepared "to go to the hilt" to isolate Russia.
Still, politicians were treading carefully, knowing it was a delicate time for Europe.
"We are on a very dangerous track of increasing tensions," German Foreign Minister Frank-Walter Steinmeier said. "(But) it is still possible to turn around. A new division of Europe can still be prevented."
So far, however, Ukraine's new government and other countries have been powerless to counter Russia's tactics. Armed men in uniforms without insignia have moved freely about Crimea for days, occupying airports, smashing equipment at an air base and besieging a Ukrainian infantry base.
Putin has defied calls from the West to pull back his troops, insisting that Russia has a right to protect its interests and those of Russian-speakers in Crimea and elsewhere in Ukraine. His confidence is matched by the knowledge that Ukraine's 46 million people have divided loyalties. While much of western Ukraine wants closer ties with the 28-nation European Union, its eastern and southern regions like Crimea look to Russia for support.
Russia has long wanted to reclaim the lush Crimean Peninsula, part of its territory until 1954. Russia's Black Sea Fleet pays Ukraine millions every year to be stationed at the Crimean port of Sevastopol and nearly 60 percent of Crimea's residents identify themselves as Russian.
During a phone conversation Sunday with German Chancellor Angela Merkel, Putin "directed her attention to the unrelenting threat of violence from ultranationalist forces (in Ukraine) that endangered the life and legal interests of Russian citizens," according to a Kremlin statement.
The statement also said "the measures taken by Russia are fully adequate with regard to the current extraordinary situation."
Ukraine's new government came to power last week following months of pro-democracy protests against a pro-Russian president, Viktor Yanukovych, and his decision to turn Ukraine toward Russia instead of the EU. Yanukovych fled to Russia after more than 80 people were killed in the protests. He insists he's still president.