On the 25th anniversary of China's crackdown on protestors in Beijing's Tiananmen Square, the government is heavily involved in censoring any online mention or celebration of the event, observers say.

Chinese authorities have been cracking down on search engines and news outlets both inside the country -- such as popular Chinese social media site Weibo -- and outside, including search company Google and media sites like the Wall Street Journal.

In China, Google's Chrome browser start page and many more Google sites could not be loaded.

"The block is indiscriminate as all Google services in all countries, encrypted or not, are now blocked in China," activist blog GreatFire.org reported. "This blockage includes Google search, images, translate, Gmail and almost all other products."

GreatFire.org called the outages "the strictest censhorship ever deployed."

Meanwhile, the Journal, which has been increasing its coverage of all things China recently, has had both its English-language and its Chinese-language sites blocked.

Facebook is not seeing any increase in censorship for the simple fact that China has blocked the popular social media site for years.

Business networking site LinkedIn has come under fire for what many are calling "self-censorships" after the site sent email to some users informing them their LinkedIn posts would not be visible in mainland China.

"This is due to specific requirements within China to block certain content so that it does not appear on our network in that country," the messages said.

The company has said it has had to agree to censor material deemed sensitive by the Chinese government in return for permission to operate in China.

LinkedIn is one of only a few American Internet companies to have received such permission from Beijing.

Some LinkedIn users slammed the site's decision.

"The way they chose to inform the users was objectionable," said Hong Kong legislatore Charles Mok, who had been posting material about Tiananmen. "And it's terrible that this has to happen exactly on [the] June 4th [anniversary]."

In addition to Internet censorship, China is reacting to the anniversary of its crackdown on the country's nascent democracy movement by reportedly arresting a number of high-profile journalists, lawyers, activists and academics.

Although Chinese users can still access Weibo, any search for terms such as "Tiananmen" or "square" or even "mourn" are blocked, with users seeing only a notice that search results cannot be shown because of "relevant laws, regulations, and policies."

China's government seems willing to keep Weibo up because the homegrown social media site is much easier for it to censor in this manner.

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