MOSCOW — As American and allied officials celebrated the opening of a long-awaited missile defense system in Europe with a ribbon cutting and a band, the reaction in Russia on Thursday suggested the system had raised the risks of a nuclear war.

Russian officials reiterated their position that the American-built system imperiled Russia’s security. But the public discussion in Russia was darker, including online commentary of how a nuclear confrontation might play out in Europe, and the prospect that Romania, the system’s host, might be reduced to “smoking ruins.”

“We have been saying right from when this story started that our experts are convinced that the deployment of the ABM system poses a certain threat to the Russian Federation,” the Kremlin spokesman, Dmitri S. Peskov, told reporters in a conference call.

“Measures are being taken to ensure the necessary level of security for Russia,” he said. “The president himself, let me remind you, has repeatedly asked who the system will work against.”