SIMFEROPOL, Ukraine — With the Russian flag planted atop the regional Parliament, Crimea raised the specter of secession from Ukraine on Thursday, threatening renewed civil conflict and a showdown between Ukraine’s fledgling government and the Kremlin.

Across the region’s capital, Simferopol, a well-orchestrated power grab by pro-Russian forces played out: Armed militants took control of government buildings; crowds filled the streets chanting “Russia, Russia,” and legislators called for a vote to redefine relations with Ukraine. The region is currently autonomous, meaning it has greater local control over its affairs.

Police officers, nominally under the control of the Ministry of Interior in Kiev, made little or no effort to control the crowds and, in some cases, even applauded their pro-Russia zeal. The police stood aside as the armed militants who seized government buildings overnight on Thursday built a barricade outside the regional legislature. The authorities ordered an emergency holiday, leaving streets mostly empty except for the protesters chanting for Russia, and many shops closed.

“This is the first step toward civil war,” said Igor Baklanov, a computer expert who joined a group of anxious residents gathered in a cold drizzle at a thin police line near the Parliament building, a line that quickly vanished when activists of a nationalist group called Russian Movement of Ukraine marched up waving Russian flags. They were followed by columns organized by Russian Bloc, another pro-Moscow organization.