The fledgling government in Kiev put the country on a war footing on Sunday as the Russian president, Vladimir Putin, tightened his grip on the Ukrainian Black Sea peninsula of Crimea and western powers were left scrambling to find a response to the escalating crisis.

"We are on the brink of disaster," said Arseniy Yatsenyuk, Ukraine's acting prime minister, as Kiev called for help from Washington and London, co-signatories of a 1994 pact with Russia guaranteeing Ukraine's security and its borders.

"This is actually a declaration of war on my country," he said. "We urge Putin to pull back his troops from this country and honour bilateral agreements. If he wants to be the president who started a war between two neighbouring and friendly countries, he has reached his target within a few inches."

As John Kerry, the US secretary of state, described Russia's gambit as "an incredible act of aggression", western powers pondered their limited options. Nato ambassadors met in Brussels, with Lithuania and Poland arguing that Russia's actions threatened them as Nato members bordering Russia and Ukraine, and pushing for appropriate action.

However, Kerry ruled out a military response. "The last thing anybody wants is a military option in this kind of situation," he said. Instead, leading western countries dropped out of preparatory work for a G8 summit hosted by Russia in June, with Kerry saying Moscow could be expelled from the organisation and economic sanctions aimed at the Russian elite, moves unlikely to have much impact.

Kiev ordered a call-up of military reserves, but also instructed its troops not to respond to Russian military "provocations" for fear of triggering a bloodbath as Russian forces in Crimea restricted Ukrainian units' movements and demanded they surrender their weapons. "The ministry of defence is calling on all the reservists. We are asking all those who have been called to show up at the mobilisation stations," said Andriy Parubiy of Ukraine's security and defence council.

As Yatsenyuk spoke, hundreds of Russian troops surrounded a Ukrainian base just outside the Crimean capital, Simferopol, in the latest military manoeuvre on the peninsula indicative of a move by Moscow to annex the peninsula in all but name.

The Guardian saw crowds of Russian civilians gathering outside the base at Perevalnoye. Russian units have already secured the parliament building in Simferopol and two airports in Crimea in moves that have provoked the gravest crisis in post-Soviet areas since the 2008 Russian-Georgian war and have the potential to turn into Europe's worst conflict since the Yugoslav wars of the 1990s.

"Their aim is to stop Ukraine's economy and to start chaos," said the acting president, Oleksandr Turchynov. "That is why they try to start panic."

Officials in Kiev said that outside Crimea, in eastern Ukraine, up to 200 Russian citizens had attacked regional administration offices in three cities. "Everyone should do everything to mobilise over the idea of an independent Ukraine," Vitali Klitschko, a Ukrainian presidential contender, told a rally in Kiev.

The first president of an independent Ukraine, Leonid Kravchuk, said: "I'm 80, but I will take my gun and will be defending my land."

A febrile pre-war atmosphere gripped Kiev on Sunday as Ukrainians turned out in their tens of thousands to denounce Russia's moves in Crimea and signal a determination to resist any further incursions from their eastern neighbour.

A day after ugly confrontations in the eastern Russian-majority cities of Kharkiv and Donetsk, inhabitants of the capital said they were ready to respond to calls for mass mobilisation from the new leaders in Kiev. "I'm a lieutenant of the reserve," said Andry Cherin, a 29-year-old scientist. "I will go to the recruitment office. I'm against war, but there are no other choices, we need to defend our country. This is our land. Russians don't need war. I think these are only Putin's ambitions."

Ukrainians were shocked by Saturday's images from Kharkiv showing Ukrainian nationals being paraded through a crowd of Russians who beat them with sticks and kicked them. There were similar scenes in Donetsk. "What Russia is doing now in Ukraine threatens peace and security in Europe," said Nato's secretary general, Anders Fogh Rasmussen.

EU foreign ministers are to meet in emergency session in Brussels on Monday, where Britain's foreign secretary, William Hague, will report on his talks in Kiev on Sunday with the interim Ukrainian government, which has been in office only a matter of days.

Berlin said it was not too late to turn back from the abyss, without proposing any decisions or action. "A turnaround is still possible. A new division in Europe can still be prevented," said Frank-Walter Steinmeier, the German foreign minister.

As the White House struggled to put pressure on Putin, Kerry accused the Russian leader of acting "in 19th-century fashion by invading another country on a completely trumped up pretext". "It is really a stunning, wilful choice by President Putin to invade another country. Russia is in violation of the sovereignty of Ukraine. Russia is in violation of its international obligations."

Senator Lindsey Graham of South Carolina, a longstanding critic of Obama's foreign policy credentials, urged the president to "do something" rather than deliver what he called empty threats to "thugs and dictators".

"Every time the president goes on television and threatens someone like Putin, everybody's eyes roll, including mine," Graham told CNN. "We have a weak and indecisive president that invites aggression."

The diplomatic flurry followed a tense 90-minute phone conversation on Saturday night between Barack Obama and Putin. The Russian president told his US counterpart that Moscow had the right to protect its interests and those of Russian speakers not only in Crimea but also in east Ukraine. Tensions escalated into the night when two Russian anti-submarine warships appeared off Crimea's coast, violating an agreement on Moscow's lease of a naval base, a Ukrainian military source was quoted as saying. The two vessels, part of Russia's Baltic fleet, had been sighted in a bay at Sevastopol, where the fleet is based.

After several days of Russian stealth, the move to deploy forces came suddenly and decisively. The Kremlin said Putin wanted troops in Ukraine "until the sociopolitical situation is normalised". Less than an hour later, in a hastily convened extraordinary sitting of Russia's Federation Council that was laced with cold war rhetoric, senators voted unanimously to support Putin's plan, and proposed withdrawing Russia's ambassador to the US.