DONETSK, Ukraine (AP) — More than 5,000 pro-Russia residents of a major city in Ukraine's east demonstrated on Saturday 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 the Western countries. After the referendum, Russia moved to formally annex Crimea.

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 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.

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 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 shield-wielding riot police. Inside, German Foreign Minister Frank-Walter Steinmeier was meeting with local officials.

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The demonstrators erected several tents, an ironic echo of the massive 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.

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 did not 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."

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.

The seizure of military facilities and navy ships by pro-Russian forces in Crimea has been proceeding apace since the peninsula was this week nominally absorbed by Russia.

On Saturday, a crowd stormed the Novofedorivka base, some 50 kilometers west of the Crimean capital of Simferopol, Ukraine's Defense Ministry said.

Ukrainian television station TSN said troops inside the base hoisted smoke grenades in an attempt to disperse groups of burly young men attempting to break through the front gates.

TSN reported that there were children among the crowd attempting to seize the base.

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.

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Jim Heintz and Peter Leonard in Kiev and Nataliya Vasilyeva in Moscow contributed to this report.