In Kiev, the Ukrainian capital, Prime Minister Arseniy Yatsenyuk said there was no reason for Russia to invade Ukraine and warned that "we are on the brink of disaster."
But so far, his new government and other countries have been powerless to react to Russian military tactics. Armed men in uniforms without insignia have moved freely about the peninsula, 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.
Russia has long wanted to reclaim the lush Crimean Peninsula, which was part of Russia until 1954. It's Black Sea Fleet is stationed there and nearly 60 percent of Crimea's residents identify themselves as Russian.
Ukraine's population of 46 million has divided loyalties between Russia and Europe, with much of western Ukraine advocating closer ties with the EU, while eastern and southern regions like Crimea look to Russia for support.
Unidentified troops pulled up to the Ukrainian military base at Perevalne on the Crimean Peninsula in a convoy that included at least 13 trucks and four armored vehicles with mounted machine guns. The trucks carried 30 soldiers each and had Russian license plates.
A dozen Ukrainian soldiers, some with clips in their rifles, placed a tank at the base's gate, leaving the two sides in a tense standoff.
Ukraine's acting president, Oleksandr Turchynov, announced late Saturday that he had ordered Ukraine's armed forces to be at full readiness because of the threat of "potential aggression." He also said he had ordered stepped-up security at nuclear power plants, airports and other strategic infrastructure.
But the U.S. and other Western governments have few options to counter Russia's military moves.
In Brussels, NATO's secretary general said Russia had violated the U.N. charter with its military action in Ukraine, and he urged Moscow to "de-escalate the tensions." NATO chief Anders Fogh Rasmussen spoke before a meeting Sunday of the alliance's political decision-making body to discuss the crisis.
Ukraine is not a NATO member, meaning the U.S. and Europe are not obligated to come to its defense. But Ukraine has taken part in some alliance military exercises and contributed troops to its response force.
Kerry, interviewed on Sunday news shows in the U.S., raised the possibility of boycotting the G-8 summit, which is to be held in June in Sochi, the Russia resort that just hosted the Winter Olympics. He also discussed visa bans, asset freezes, and trade and investment penalties. Kerry said he spoke with foreign ministers for G-8 and other nations on Saturday, and says everyone is prepared 'to go to the hilt" to isolate Russia.
President Barack Obama spoke with Putin by telephone for 90 minutes Saturday and expressed his "deep concern" about "Russia's clear violation of Ukrainian sovereignty and territorial integrity," the White House said. Obama warned that Russia's "continued violation of international law will lead to greater political and economic isolation."
In Moscow, thousands marched Sunday in a pro-invasion rally one day after Russia's parliament gave Putin a green light to use military force in Ukraine. At least 10,000 people bearing Russian flags marched freely through the city, while dozens of people demonstrating on Red Square against an invasion of Ukraine were quickly detained by Russian riot police.
The new Ukrainian 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 European Union. Yanukovych fled to Russia after more than 80 people died, most of them demonstrators killed by police. He insists he's still president.
Since then, tensions have risen sharply between the two capitals.
The Interfax news agency reported the speaker of Crimea's legislature, Vladimir Konstantinov, as saying the local authorities did not recognize the government in Kiev. He said a planned referendum on March 30 would ask voters about the region's future status.
The White House said the U.S. will suspend participation in preparatory meetings for the Group of Eight economic summit planned.
French Foreign Minister Laurent Fabius said on Europe 1 radio that planning for the summit should be put on hold. France "condemns the Russian military escalation" in Ukraine, and Moscow must "realize that decisions have costs," he said Sunday.
"We are on a very dangerous track of increasing tensions," German Foreign Minister Frank-Walter Steinmeier said in a statement. "(But) "it is still possible to turn around. A new division of Europe can still be prevented."
McHugh reported from Kiev, Ukraine. AP correspondent Greg Keller contributed from Paris.