Armed men take up positions around the regional parliament building in the Crimean city of Simferopol

SIMFEROPOL Ukraine — As Russian-backed armed forces effectively seized control of Ukraine’s Crimean peninsula on Saturday, President Vladimir V. Putin of Russia requested that the Russian Senate authorize him to use military force in Ukraine.

Mr. Putin’s request, largely a formality, signaled publicly for the first time the Kremlin’s readiness to intervene militarily in Ukraine, and it served as a blunt response to President Obama, who just hours earlier pointedly warned Russia to respect Ukraine’s sovereignty.

Even as Mr. Putin submitted his request to the Senate, formally called the Federation Council, it was clear that forces allied with Moscow were largely in control of the disputed peninsula.

Just a few hours earlier on Saturday morning, the newly installed, pro-Russia prime minister of Crimea declared that he had sole control over the military and the police in the disputed peninsula and he appealed to Mr. Putin for help in safeguarding the region.

The prime minister, Sergei Aksyonov, also said a public referendum on independence would be held on March 30.

On a day of frayed nerves and set-piece political appeals that recalled ethnic conflicts of past decades in the former Soviet bloc — from the Balkans to the Caucasus — pro-Russian forces were said to have taken control of a government building in Kharkiv, and a crowd in the center of Donetsk pulled down the blue-and-yellow Ukrainian flag and raised a Russian one.