Maoflag.net is among the leftist sites that has been closed to internet users in China. This screengrab is from March 18, 2012, taken using archive.org. (Screenshot from Maoflag.net)

China’s Communist Party censors on Friday closed several “new left” Web sites, and a pro-reform site run by the Carter Center went offline, as the country’s rulers sought to stifle divergent voices and muffle signs of an ideological struggle ahead of a crucial leadership change this fall.

The Web sites Maoflag.net, Jinbushe.org (or “Progress Society”) and wyzxsx.com, the Internet home of Utopia, a neo-Maoist group, were among those closed to Internet users in China. All three have been outspoken in their support of the popular and charismatic former Chongqing party chief Bo Xilai, who was fired March 15. They were also cheerleaders for Bo’s “Chongqing model” of development, which emphasized a strong state-centered economy and social welfare policies to address income inequality.

The crackdown was the latest step in an ongoing tightening of Internet controls since Bo’s dismissal, as top party officials seek to contain the public fallout from China’s most serious political crisis in decades. Web search terms have been blocked and other sites shut down, and some of Bo’s past associates have been removed from their posts and placed under investigation.

On Friday, the Maoflag and Utopia Web sites initially carried an identical message saying they had been shut down for a month by the State Council Information Office and the Public Security Bureau for “posting articles and information that are anti-constitution, maliciously attacking national officials, and spreading rumors about 18th Party Congress.” That message was later taken down, replaced by a note saying they were having technical problems.

The 18th Congress, set for this fall, is due to select a new party secretary and president — now expected to be the current vice president, Xi Jinping — and seven of nine new members on the powerful Politburo Standing Committee, which effectively runs the country.

Another leftist site, redchinacn.com, was also closed late Friday, and comments were disabled on the nationalist news site m4.cn, originally known as the “anti-CNN” Web site.

In a telephone interview, Rao Jin, the founder of m4.cn, said the site’s bulletin board was being revised to include more lifestyle and entertainment features. He acknowledged, “We do face very big pressure now,” but added, “it’s not convenient to tell you the details.”

On the opposite side of the political spectrum, the Chinese-language Web site of China Elections and Governance, which is run by the Carter Center and advocates political reform, also was inaccessible Friday. A notice said the site was experiencing technical problems.

“The party doesn’t want too much noise before the 18th Party Congress,” said Chen Yongmiao, an independent political observer in Beijing. “The rightists want political reform. The leftists want social welfare” and a more equitable distribution of wealth. “The party just wants the two sides to be quiet and stop talking about politics.”

Until his firing last month, Bo — the son of revolutionary hero Bo Yibo — was widely considered to be in line for one of the nine Standing Committee positions. His removal has not been officially explained, leaving an information vacuum that is being filled with Internet-fueled rumors, including unsubstantiated claims that he is under investigation for corruption or that his popular following unnerved a leadership that favors faceless, collective decision making.

The Internet closures suggest that three weeks after Bo’s ouster, the ideological struggle continues, with the government and ruling party on edge about any suggestion of dissension in the top ranks. In a telling sign of the mood, the daily newspaper of the People’s Liberation Army on Friday ran the latest of several recent editorials calling for unity and reiterating the military’s allegiance to President Hu Jintao.

The editorial mentioned the upcoming 18th Party Congress as a major event for China. “Whenever the party and nation are facing big events, whenever the process of reform and development is reaching a crucial point, the ideological battle becomes sharper and more complicated,” it said.

The editorial then warned: “We must unswervingly adhere to the system and principle of the absolute leadership of the party over the army. We must unswervingly resist all kinds of wrong thoughts, not be disturbed by noise, not be confused by rumors, not be moved by dark currents, and ensure the troops listen to the command of the Central Military Commission and the Party Central Committee and Chairman Hu, at any time and under any circumstances.”

The editorial appeared to be a direct response to unsubstantiated talk of a military coup in the making following Bo’s ouster. On March 30, government censors announced a half-dozen arrests and the closing of 16 Web sites for “fabricating or disseminating online rumors,” including tales of tanks in the streets of Beijing.

Bo Xilai had become a hero to China’s relatively small but outspoken circle of far-left academics, writers, bloggers and students for his emphasis on social welfare and his unabashed imitation of Mao Zedong’s “mass mobilization” style of politics. In Chongqing, Bo instituted a “red revival” campaign that included organized singing of revolutionary songs. In other throwbacks to the Mao era, he also ordered Chongqing satellite television to replace prime-time sitcoms and game shows with “patriotic” programming and directed students and government workers to spend time in rural areas.

But it was Bo’s economic policies in Chongqing that most endeared him to the “new leftists,” as they are known here. For example, Bo invested heavily in the construction of subsidized housing for low-income residents, with cranes visible all across the sprawling city.

In a March 14 news conference, Prime Minister Wen Jiabao, considered the leading if lonely voice of Chinese “reformists” calling for more economic and political openness, seemed to directly challenge Bo’s vision for China’s future. Without continued opening and reform, Wen warned, “historic tragedies like the Cultural Revolution may happen again.”

Wen has kept up his calls for Western-style economic liberalization since Bo’s ouster, angering the new leftists. On Tuesday, he urged a breakup of China’s powerful banking monopoly, saying the large state-run banks mostly benefit powerful state-owned corporations at the expense of small and medium-size private businesses.

The new leftists, in response, posted articles on their Web sites calling for preservation of the state monopolies and praising Bo’s Chongqing model. They say now that their voices are being silenced, as Wen and the reformers seize the upper hand in the wake of Bo’s firing.

“The authorities have deprived the leftists of their free speech rights,” said Zhang Hongliang, an economics professor at Beijing’s Minzu University of China and an active leftist who regularly attends Utopia functions.

“In China, the leftists don’t have their own newspapers, television or radio. A few Web sites are our only channel to get our voices out,” he said. “Right now, our Web sites are closed.”

Researchers Zhang Jie and Liu Liu in Beijing contributed to this report.