An APC also smashed open the front gate of the Belbek base near the port city of Sevastopol, according to footage provided by the Ukrainian Defense ministry. Two ambulances arrived and then departed shortly after, and at least one of them was carrying what appeared to be a wounded person.
The Ukrainian commander of the base, Col. Yuliy Mamchur, said there was at least one injury. He called his men together, they sang the Ukrainian national anthem and then stood at ease. He then told his men to put their weapons in the base's armory.
The men who stormed the base didn't wear any insignia. A Defense Ministry spokesman, Vladislav Seleznev, said on Facebook that they were part of the local militias that have been formed over the past several weeks, but their machineguns and APCs appeared to indicate a military connection.
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.
Also Saturday, a crowd stormed the Novofedorivka base, about 50 kilometers (30 miles) west of Simferopol, Ukraine's Defense Ministry said.
Ukrainian television station TSN said troops inside the base hurled smoke grenades in an attempt to disperse groups of burly young men attempting to break through the front gates.
Elsewhere, more than 5,000 pro-Russia residents of a major city in Ukraine's east demonstrated in favor of holding a referendum on whether to seek to split off and become part of Russia.
The rally in Donetsk came less than a week after the Ukrainian region of Crimea approved secession in a referendum regarded as illegitimate by Western countries. After the referendum, Russia formally annexed Crimea.
With Crimea now effectively under the control of Russian forces, which surround Ukrainian military bases on the strategic Black Sea peninsula, concern is rising that Ukraine's eastern regions will agitate 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.
However, Valentina Matvienko, the speaker of the upper house of the Russian parliament and a close Putin ally, said Saturday that there is no intention to absorb other regions of Ukraine, according to the ITAR-Tass news agency.
Eastern Ukraine is the heartland of Ukraine's economically vital heavy industry and mining. It's also the support base for Viktor Yanukovych, the Ukrainian president who fled to Russia last month after three months of protests in the capital, Kiev, triggered by his decision not to sign an agreement with the European Union.
Russia and Yanukovych supporters contend Yanukovych's ouster 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.
"They're trying to tear us away from Russia," said demonstrator Igor Shapoval, a 59-year-old businessman. "But Donbass is ready to fight against this band which already lost Crimea and is losing in the east."
Donbass is the name for the region of factories and mines that includes Donetsk.
About an hour after the Donetsk rally began, the crowd marched through the city center and assembled before the regional administration building chanting: "Crimea! Donbass! Russia!"
Demonstrators waving Russian flags were faced off by lines of riot police. Inside, German Foreign Minister Frank-Walter Steinmeier met with local officials.
"We heard that there is much desire to preserve Ukraine as a single state," he told reporters after the meeting.
The demonstrators erected several tents, an ironic echo of the huge tent camp that was established on Kiev's central square after the protests against Yanukovych broke out in late November.
"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 analogous to the one in Crimea. Activists on Saturday passed out mock ballots, 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 Saturday that they have arrested Mikhail Chumachenko, leader of the self-styled Donbass People's Militia, on suspicion of seeking to seize authority.
U.N. Secretary-General Ban Ki-moon met with Ukraine's acting prime minister, Arseniy Yatsenyuk, in Kiev on Saturday and said "I am confident that with such strong support from the international community that you are receiving, under your leadership as well as with your courageous people, you will be able to overcome this difficult time."
Sanctions imposed this week by the U.S. and the European Union haven't persuaded Russia to back off on its intent to annex Crimea.
As tensions roil in the east, the Organization for Security and Cooperation in Europe is deploying an observer team aimed at easing the crisis.
Russian Foreign Ministry spokesman Alexander Lukashevich said in a statement on Friday that Moscow hopes that the 200-strong team "will help to overcome the internal Ukrainian crisis" and ensure the respect for human rights there.
It is unclear whether the team will be allowed into Crimea. Russian forces last week stopped OSCE military observers from entering Crimea. The organization on Friday didn't specify whether the observers will go to Crimea.
Lukashevich said on Saturday 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."
Sevastopol, a city in southwest Crimea, is the home of Russia's Black Sea Fleet.
Daniel Baer, the chief U.S. envoy to OSCE, said the observers should have access to the territory because Crimea remains Ukrainian to the rest of the world.
The seizure of military facilities and navy ships by pro-Russian forces in Crimea has been proceeding apace since the peninsula was nominally absorbed by Russia.
The Russian Defense Ministry says that as of late Friday less than 2,000 of 18,000 Ukrainian servicemen in Crimea had "expressed a desire to leave for Ukraine." The ministry, however, stopped short of saying the remainder of the troops would serve in the Russian army.
Jim Heintz and Peter Leonard in Kiev and Nataliya Vasilyeva in Moscow contributed to this report. Karmanau reported from Donetsk, Ukraine.