Chinese President Xi Jinping is aiming to clean up the Communist Party from within while silencing external critics. (EDGARD GARRIDO/REUTERS)

The trial of Bo Xilai may have divided the Chinese Communist Party and hogged the media spotlight, but outside the courtroom, President Xi Jinping is continuing his steady efforts to consolidate control, clean up the party and sideline opponents, with a series of detentions and arrests in the past few days.

On Monday, a senior manager of the giant oil company China National Petroleum was placed under formal investigation for “severe breaches” of Communist Party discipline, the government announced, the second executive of a major state-owned enterprise to face such an inquiry this month. Xi has vowed to fight the vested interests that are thought to be blocking efforts to reform the country’s economy, as well as to eliminate the “tigers” and the “flies” — senior and junior officials — believed to be responsible for widespread corruption.

Wang Yongchun was vice president of China National Petroleum, the parent company of PetroChina; he was also the head of the country’s biggest oil field, Daqing, and was seen as a likely candidate to take over the company earlier this year. A regional manager of China Mobile was placed under a similar investigation this month.

The party’s efforts to assert more control over social media have also intensified. On Sunday, authorities announced the detention or arrest of three prominent critics: A 60-year-old Chinese American businessman, Xue Manzi, a widely followed commentator on social media, was detained in Beijing on Friday night on suspicion of soliciting a 22-year-old prostitute; a newspaper reporter who had accused a senior politician of corruption was detained in Chong­qing; and a whistleblower was arrested for allegedly soliciting bribes from people he had accused. Two other leading bloggers were also detained last week as part of a campaign against people accused of spreading “rumors” online.

Political analysts in China said the Wang investigation could be an attempt to limit the power of an ally of Bo, whose trial on charges of corruption, embezzlement and abuse of power concluded Monday with closing arguments from both sides.

Ever since Bo’s fall from grace, there has been speculation that one of his strongest and most powerful supporters, former security chief Zhou Yongkang, could be the next to be ousted. While it is considered unlikely that Xi would want to directly take on someone as senior as Zhou, who served until last year as a member of the party’s elite Standing Committee, Wang’s arrest could be an attempt to clip his wings. Zhou was widely seen as controlling the state’s monopoly of the oil sector.

“The aim of this operation is to separate Zhou Yongkang from the current political layout,” said Li Weidong, a political commentator and former editor of China Reform magazine. “This is not a swipe at the tiger but meant to transform the tiger into a cat.”

Hu Xingdou, an economics professor at the Beijing Institute of Technology, said Xi’s anti-corruption campaign is more intense than many previous attempts to clean up the party from within and has caused concern among local officials. But he said that an “iron fist” would have only short-term results and catch a small proportion of the corrupt. Only by freeing the media or passing a law forcing officials to disclose their assets could real progress be made, he said.

But further evidence that this sort of reform is not in the cards came over the weekend with the latest attack on the government’s critics, among them Xue, whose liberal posts had won him 12 million followers on Sina Weibo, China’s version of Twitter.

On Sunday, the editor of the state-run Global Times newspaper suggested on weibo that the authorities might be deliberately giving Xue a “hard time” for his political views.

But by Monday, the newspaper seemed to have backtracked, warning in an editorial that “influential” figures must expect close scrutiny. “People who are addicted to raising up political confrontations should be clear that without a clean conscience, they are engaging in a groundless undertaking,” it wrote.

Earlier this month, the government called in a group of the country’s most widely followed social-media commentators and urged them to be more constructive in their postings.

Liu Liu contributed to this report.