BEIJING — China’s formidable propaganda apparatus came under renewed attack on Friday, when a denunciation spread online in the name of an employee of Xinhua, the main state-run news agency. The letter accused censors of using tactics reminiscent of Maoist times to silence and smear critics.

The letter reflected a growing discontent among journalists, academics and even party insiders about the tighter censorship and about the giddy exultation of President Xi Jinping in state-run media.

“Under the crude rule of the Internet control authorities, online expression has been massively suppressed, and the public’s freedom of expression has been violated to an extreme degree,” said the letter, which spread quickly online in China and was taken down just as swiftly.

The letter was issued in the name of Zhou Fang, who gave his work address as Xinhua News Agency headquarters in Beijing, and included his cellphone number and identity card number. A man who answered the phone at that number said that he was Mr. Zhou, an employee of Xinhua, and that he had written the letter.