The Russians arrived at the base early in the morning with a blunt message: give up your weapons within the hour or face the consequences. But the elite Ukrainian battalion, based outside the Crimean town of Feodosia, told them politely but firmly to get lost.

"We gave an oath to the state of Ukraine, not an oath to one particular general, and certainly not one from another country," Major Rostislav Lomtev told the Guardian from inside the base. The Russians duly disappeared. But not for long. They would be back later. The resolution to this standoff remains precariously inconclusive.

If Saturday was the day when Vladimir Putin won official backing from his parliament to introduce troops to Ukraine, Sunday was when Russia mobilised its military to attempt to win the new Crimean war without firing a single bullet.

Across the peninsula, Russian armoured personnel carriers arrived at Ukrainian bases and pressured the inhabitants to give up their arms, as politicians in Kiev spoke of an "invasion" and the west looked on in horror. The head of the navy, Denis Berezovsky, appeared on television to announce he was defecting to the pro-Moscow Crimean separatists, though he was quickly fired and accused by Kiev of treason. Elsewhere, for now, the Ukrainian military appeared to be holding strong.

Ukrainian marines guard the gates of their base in Feodosia. Viktor Drachev/AFP/Getty Images

It was still not clear exactly what the Kremlin's goal was in Ukraine, though increasingly the signs seemed to point to an annexation of Crimea, at the very least. Checkpoints manned by irregulars remained on major roads, and troops were spotted digging trenches at the narrow entrance to Crimea from the rest of Ukraine. Across the peninsula, soldiers executed pinpoint visits to Ukrainian military facilities, demanding that the Ukrainians give up their weapons and pledge their allegiance to the new authorities in Crimea.

At Feodosia, where the Guardian was given exclusive access to the inside of the base, the marines created a small barricade of tyres and barbed wire at the entrance to the base after the Russian ultimatum in the morning, and built another barricade of sandbags in front of the main entrance of the two-storey white stone headquarters.

Outside, a group of Russians gathered, waving Russian and Crimean flags. Appropriating the tropes of the Kiev anti-government protests, women handed out tea and sandwiches, and people sang songs to the accompaniment of an accordion. The mood was positive, with chants of "Russia! Russia!" but there was an undercurrent of aggression, especially when a group of the Russians discovered a western reporter in their midst.

Military personnel stand near a Russian-made Kamaz truck in the eastern Crimea'a port city of Feodosia. Photograph: Viktor Drachev/AFP/Getty Images

There were also three armoured personnel carriers and several dozen troops standing guard. They had been here for days, despite the Kremlin only voting to send troops in on Saturday. They are, if the Russian version of events is to be believed, the best-equipped local volunteer brigades in the world.

But it would take an extraordinarily naive person to believe that. The troops may not wear insignia on their uniforms, but they arrived in trucks with Russian military plates and are well armed. Privately, the commanders admitted to Ukrainian troops that they were Russian regular troops. Given that some of them were personally acquainted, there was not much point in hiding it.

As Major Lomtev in Feodosia put it: "We know a lot of them, because we have done a lot of joint exercises together and have close links with them. To be honest, I don't think they are very proud of the orders they are carrying out at the moment."

At Perevalnoye, not far from the regional capital, Simferopol, more than a dozen trucks stood nearby as more than 100 soldiers patrolled the perimeter and nearby hills. The Ukrainian coastguard division inside said they would not give up the base, staring out at the Russians from behind the gate.

After negotiations, the two sides agreed not to point their guns at each other, but the standoff remained tense, with a priest from the nearby Ukrainian orthodox church reciting prayers and brandishing a cross.

A large group of local people gathered outside the base waving the Russian tricolour flag and chanting "Russia! Russia!" A lone, middle-aged woman shouted "Glory to Ukraine", the slogan of the protest movement in Kiev, and was met with angry catcalls.

"Fuck off with your disgusting Ukraine," shouted one of the men, and she scurried off in distress. "I am so sad, so sad for Ukraine, it hurts to watch all of this, it's such an obvious provocation," she said, fighting back tears.

But Russia is not short of support on the ground in Crimea. Alexander Minayev, outside the base at Perevalne, said: "There are people who are openly Nazis in the new government in Kiev. They attacked police, they set them on fire. In what other country would that be OK? I know that you westerners with your disgusting media will turn everything upside down, but thank God Putin is here to save us."

Crimea's leaders have called for a referendum that would extend the territory's autonomy, to be held on 30 March. In Simferopol, the local parliament speaker, Vladimir Konstantinov, refused to say whether he still accepted the Ukrainian constitution, and was evasive about whether the territory might later appeal to join up with Russia.

Military personnel stand guard in the Crimean port city of Feodosia. Photograph: Thomas Peter/Reuters

There was confusion, too, on the ground. Inside the base at Feodosia, word came that the order had come from Kiev that all marines should give up their weapons voluntarily to the Russians. One of the marines then read on Facebook that the news was apparently fake, and rushed off to inform his superiors who were in negotiations with Russians. Many of the marines were on the phone to friends at other bases, asking whether they had been forced to give up their weapons.

Russian state media reported widespread desertions to the Crimean forces. It was difficult to verify whether this really was happening. Even an official representative of the Ukrainian defence ministry seemed unclear. Ruslan Semenyuk said: "I don't know of any cases where soldiers have voluntarily given up their arms, but there is one base where the Russians seized the weapons, and then moved them to another Ukrainian base. We don't know why they did this, maybe just as a display of force. About other cases, I don't know."

At Feodosia, the Russians came back, led by a lieutenant-general and a retired general, for more talks. After meeting the commanders, a group of three dozen officers assembled to be briefed by the Russians. The Guardian was asked to leave the room during the talks, as the Russians refused to speak in front of journalists. Afterwards, the Ukrainians said they had again been asked, "firmly but politely" to give up their weapons, and had again refused.

One marine, carrying his kalashnikov through the grounds of the base late in the evening, did not want to be identified but was happy to share his allegiances.

Asked what would happen if, next time, the Russians came with orders rather than requests, the men claimed to have no doubt. "I am Russian myself, I was born there," he said. "But we are professional soldiers and we have given an oath of duty. We will not give up this place without a fight."