Armoured vehicles smashed through metal gates and concrete walls as pro-Russian forces fired shots in the air and used stun grenades while storming a Ukrainian air force base in Crimea.
At least one Ukrainian serviceman was wounded during the assault on Belbek base, near Sevastopol, which is now said to be under Russian control.
Earlier, several hundred protesters also seized a Ukrainian naval base at Novofedorivka in western Crimea, the attackers throwing smoke bombs as they advanced.
"All is in smoke. We retreated to the main headquarters," a spokesman for the Ukraine defence ministry's Crimean region, Vladyslav Seleznyov, was quoted as saying by Ukrainian media.
In Belbek, the commander of the base Colonel Yuliy Mamchur, confirmed that a Ukrainian serviceman had been injured and that he himself was being taken away by the Russians for talks at an unspecified location.
Asked if he thought he would return safely, he said: "That remains to be seen. For now we are placing all our weapons in the base's storage."
Earlier, an unidentified Russian officer with no rank insignia had gone to the fence to try to negotiate a surrender, but had been rebuffed by a Ukrainian officer with the words: "We have no documents from the Ukrainian president stating that this is Russian soil. Bring me such a document and we will leave."
Belbek was one of the last military facilities in Crimea still under Ukrainian control following Russia's armed takeover and subsequent annexation of the peninsula, which has a majority ethnic Russian population and is home to one of Russia's biggest naval bases.
Earlier, the deputy commander of the base, Oleg Podovalov, said the Russian forces surrounding the base had given the Ukrainians an hour to surrender.
After the Russians entered, a Ukrainian officer who identified himself only as Vladislav said: "We did not provoke this, it was brute force. I do not know whether this base will be formally in Russian hands by the end of the day. Ever since world war two, this place has been quiet, and they came in here firing, with armoured personnel carriers (APCs) and grenades. I am very worried now."
Mamchur, the commander, told his troops he would inform the high command they had stood their ground. The soldiers applauded, chanting "Long live Ukraine!"
Many stood to take pictures of each other in front of the Ukrainian flag, which continued to fly over the base.
Ukraine's Defence Ministry said on Friday that Crimea's bases were still formally under Ukrainian control, but most are now occupied by Russian troops and fly Russia's tricolour flag.
Russian forces have been seizing Ukrainian military facilities for several days in the Black Sea peninsula, which voted a week ago to secede and join Russia. The fall of the Belbek base marks a serious loss for the Ukrainian military, which had been battling to keep control of the region.
Yesterday in the Crimean capital, Simferopol, a joint funeral was held for a Ukrainian soldier and a Russian Cossack killed last week in a shooting - the only other known victims of the otherwise bloodless takeover of Crimea by Russia.
The van carrying their coffins was escorted by bikers flying Russian and Crimean flags. The Crimean authorities are still investigating the shooting at a base in the city on Tuesday
Elsewhere, more than 5000 pro-Russia residents of the city of Donetsk in Ukraine's east demonstrated in favour of holding a referendum on whether to seek to split off and become part of Russia.
With Crimea now effectively under the control of Russian forces, which ring Ukrainian military bases on the strategic Black Sea peninsula, concern is rising that Ukraine's eastern regions will push for a similar move.
Russia has brought large military contingents to areas near the border with eastern Ukraine. Russian President Vladimir Putin has said there is no intention to move into eastern Ukraine, but the prospect of violence between pro and anti-secession groups in the east could be used as a pretext for sending in troops.
Eastern Ukraine is the heartland of Ukraine's economically vital heavy industry and mining and the support base for Viktor Yanukovych, the Ukrainian president who fled to Russia last month after being ousted in the wake of three months of protests in the capital, Kiev.
Russia and supporters of the former president contend that Yanukovych's ousting was a coup and allege that the authorities who then came to power are nationalists who would oppress the east's large ethnic Russian population.
About an hour after the Donetsk rally began, the crowd marched through the city centre and assembled before the regional administration building chanting: "Crimea! Donbass! Russia!"
Demonstrators waving Russian flags were faced off by lines of riot police wielding shields. Inside, German Foreign Minister Frank-Walter Steinmeier was meeting local officials.
Outside, the demonstrators put up several tents. "I'm ready to live in a tent, but I'm not ready to submit to the West, to dance to their tune," said Viktor Rudko, a 43-year-old miner.
The local parliament on Friday formed a working group to develop a referendum similar to the one in Crimea. Activists yesterday passed out mock ballot papers, although no referendum has been formally called.
A number of leading pro-Russian activists have already been detained by police on suspicion of fomenting secessionist activities. The country's security services said they had arrested Mikhail Chumachenko, leader of the self-styled Donbass People's Militia, on suspicion of seeking to seize authority.
As tensions grow in the east, the Organisation for Security and Co-operation in Europe (OSCE) is deploying an observer team aimed at easing the crisis.
Russian Foreign Ministry spokesman Alexander Lukashevich said on Friday that Moscow hoped the 200-strong team "will help to overcome the internal Ukrainian crisis" and ensure the respect for human rights there. However, it is unclear whether the team will be allowed into Crimea.
Lukashevich said yesterday that the OSCE's mission "will reflect the new political and legal order and will not cover Crimea and Sevastopol which became part of Russia".
Daniel Baer, the United States' chief envoy to OSCE, said the observers should have access to the territory because Crimea remains Ukrainian to the rest of the world.
On Friday, extravagant firework displays were staged in Crimea and Moscow to mark the formal unification of the peninsula with Russia, which Kiev and Western leaders refuse to recognise and have answered with sanctions.
Meanwhile, Moscow has described further EU sanctions against prominent Russians over its annexation of Crimea from Ukraine as "detached from reality".
The Foreign Ministry said Russia reserved the right to an "appropriate response" after Brussels last week expanded its blacklist from 21 to 33 names. Those on the list, which includes close allies of President Vladimir Putin, face asset freezes and travel bans.
Following a week of crisis, consisting largely of warnings, diplomatic wrangling and sanctions, Russia's storming yesterday of the bases in Ukraine has once again raised the spectre of a possible escalation in violence.
Ukrainian residents are mostly staying behind closed doors and some are packing their bags and preparing to leave the Black Sea peninsula.
In a small cafe in the old quarter of the regional capital Simferopol yesterday, a group of Ukrainians gathered to watch the latest news and to discuss their future.
"We are ready, packed and we can even leave tomorrow if we decide so," said Sergey, a 64-year-old Ukrainian businessman who has been selling kitchen appliances across Crimea. He refused to give his full name, fearing reprisals.
A native of the eastern Ukrainian city of Kharkiv, Sergey moved to Simferopol three decades ago and got married there.
"The good days in Crimea are over," he said, sipping coffee. "I will try to sell my business in the coming days to cut my losses and leave."