A Chinese man and woman use their smartphones in Beijing, China. China is the largest smartphone market in the world, although the country regularly imposes restrictions on internet usage. (Kevin Frayer/Getty Images)

Google has been steadily strangled, and Gmail finally blocked more effectively than ever. Instagram and Flickr recently went black, while Microsoft Outlook was hacked. In the past few days, virtual private network (VPN) services, the tools that many people use here to evade online censorship, came under renewed attack.

Brick by brick, China is building its Great Firewall steadily higher, experts say. It infuriates netizens, exasperates foreign business executives, and appears to contradict China’s pretensions to be a global superpower — and its celebrated opening to the outside world.

“China’s long-term goal is to make the Internet act like an intranet, cutting off access to all encrypted sites, so that government bureaucrats can tap into anything that anyone is saying, at any time,” said one foreign IT executive, who spoke on the condition of anonymity because of the sensitive nature of the subject.

Yet Chinese IT companies, behind the wall, are flourishing, and e-commerce is booming. As dissent is crushed, the policy’s success in consolidating Communist rule suggests that there will be no turning back.

The party’s determination to seize the “commanding heights” of the Internet here was cemented by a high-speed train crash in Wenzhou in July 2011, when news broke on social media and criticism of the authorities went viral, said Jeremy Goldkorn, director of Danwei, a media and Internet consulting firm in Beijing.

The 2009 “Facebook revolution” in Iran, and the role of social media in the Arab Spring, contributed to that determination, he said, while efforts to control the Internet have intensified amid a broader crackdown on free speech under President Xi Jinping, who assumed office in 2013.

The screws were further tightened in the second half of last year, after Xi took control of a new supervisory body designed to bolster the nation’s cybersecurity.

Hacking attacks have intensified, with users of Microsoft Outlook, Google, Yahoo and Apple services all targeted in the past three months, said Charlie Smith of GreatFire.org, an organization dedicated to combating online censorship in China. Services including Flickr, Instagram and Microsoft One Drive were blocked, as well as popular Asian chat services such as Line and Kaokao Talk.

“Last year’s crackdown has been the most aggressive in the history of Chinese censorship on the Internet,” Smith, who uses a pseudonym for security reasons, wrote in an e-mail. “The authorities now not only just target public information sharing (Facebook, Twitter, YouTube, etc.) they target private communications as well (Gmail, Outlook, IMs, etc).”

The assault on Google stretches back to 2009 and 2010, when the company finally decided not to play along with Chinese demands to censor results from its search engine. But it has intensified in recent months, with access to Google’s search engine almost completely cut, and the closing late last month of a loophole that had allowed many people to access Gmail through third-party mail services.

The move frustrated many Chinese netizens, including students who used Gmail to file their applications to colleges abroad, said William Long, the pen name of a well-known blogger on IT issues, whose article calling the blocking of Gmail a “historical regression” was widely circulated here.

While larger foreign companies spend significant sums on dedicated Internet lines that evade the restrictions, smaller firms are more affected by each tightening of controls. Goldkorn called the disruptions to Gmail “a major inconvenience that has made my business less efficient.”

Last week, Astrill, a popular VPN provider whose service allows people to jump the Great Firewall, told subscribers that its product was no longer working on iPhones and iPads.

The attacks on foreign e-mail and VPN services, it told its clients, were just a way for China to say, “We don’t want you here.” In a second, widely circulated statement, Astrill added, “We know how access to unrestricted Internet is important for you, so our fight with Chinese censors is not over.”

Another provider, Golden Frog, also reported disruptions last week — although other VPN services are working, and Astrill still functions on laptops, albeit intermittently.

In 2012, China temporarily cut VPN access almost entirely, and it has not taken such an extreme step this time; it is unclear whether that is because government officials widely use these services, or because it fears a public backlash.

Either way, last week’s attack on VPN providers provoked protests online, with netizens saying China had “lost the right to mock North Korea,” and complaining that Xi sent his daughter to the United States for an education but blocked his people from accessing information.

“Welcome to the West DPRK,” said one user, referring to the Democratic People’s Republic of Korea.

Frustration also is mounting among foreign business executives. The restrictions “massively slow Internet connection speeds,” and filter out huge amounts of non-illegal content, the European Chamber of Commerce complained last year, affecting the ability of individuals and businesses to conduct research, acting as drag on business, and diminishing quality of life.

China ranks 93rd in the world for peak Internet connection speeds, with connectivity rates less than a quarter of those in Hong Kong, according to a report by cloud computing company Akamai.

The American Chamber of Commerce in China says that “excessive control over email and Internet traffic” affects legitimate business and hampers the sharing of ideas and innovation. “That is not something in China’s best interest,” Chamber Chairman James Zimmerman said.

But it is IT companies in the United States that suffer the most from missed opportunities in China’s booming market. Even those like Microsoft that have tried to play by China’s rules have fallen out of favor here, with Beijing banning the use of Windows 8 on new government computers last year and pressing ahead with plans to develop a rival operating system.

Yet behind the Firewall, Chinese companies including Baidu, Tencent and Alibaba have thrived, drawing in talent and taking their place as some of the world’s biggest Internet firms.

The number of people online rose to nearly 650 million in 2014, many on mobile devices: Hundreds of millions are active on social media. E-commerce is booming, and powering a much-needed rebalancing of the Chinese economy toward domestic consumption. Last year, the sector’s dominant player, Alibaba, staged the largest initial public offering in history, raising $25 billion on Wall Street.

Bill Bishop, editor of the influential Sinocism newsletter, said the “gilded cage” constructed around the Chinese Internet has proved enormously profitable. At the same time, attempts to organize protests through social media have been crushed, and the faintest echo of the Arab Spring silenced.

“From this perspective, where is the problem with the policy?” Bishop asked.

Xu Jing contributed to this report.