BEIJING — Declaring that Communist Party control over Chinese media is “unshakable,” the government’s main propaganda organ appeared to take a hard line Tuesday against anti-censorship protesters at the offices of the Guangdong newspaper Southern Weekly, and warned darkly that mostly unnamed foreign agitators were behind the unrest.

The “urgent memo” from the ruling party’s Central Propaganda Department was sent to media heads and local party chiefs. It was obtained and translated by the Hong Kong newspaper South China Morning Post and the website China Digital Times, which regularly publishes edicts from China’s censorship authorities and is derisively called the “Ministry of Truth.”