SAN FRANCISCO (Reuters) - Twitter announced Thursday that it would begin restricting Tweets in certain countries, marking a policy shift for the social media platform that helped propel the popular uprisings recently sweeping across the Middle East.

"As we continue to grow internationally, we will enter countries that have different ideas about the contours of freedom of expression," Twitter wrote in a blog post published Thursday.

It said even with the possibility of such restrictions, Twitter would not be able to coexist with some countries. "Some differ so much from our ideas that we will not be able to exist there," it said.

Twitter gave as examples of restrictions it might cooperate with "certain types of content, such as France or Germany, which ban pro-Nazi content."

A Twitter spokeswoman declined to elaborate on the blog.

"Starting today, we give ourselves the ability to reactively withhold content from users in a specific country while keeping it available in the rest of the world," the Twitter blog said.

Twitter's decision to begin censoring content represents a significant departure from its policy just one year ago, when anti-government protesters in Tunisia, Egypt and other Arab countries coordinated mass demonstrations through on the social network and, in the process, thrust Twitter's disruptive potential into the global spotlight.

As the revolutions brewed last January, Twitter signaled that it would take a hands-off approach to censoring content in a blog post entitled "The Tweets Must Flow."

"We do not remove Tweets on the basis of their content," the blog post read. "Our position on freedom of expression carries with it a mandate to protect our users' right to speak freely and preserve their ability to contest having their private information revealed."

And last year, Twitter General Counsel Alex Macgillivray declared that the company was "from the free speech wing of the free speech party."

In the interest of transparency, Twitter said Thursday, it has built a mechanism to inform users in the event that a Tweet is being blocked.

Twitter's move comes at a time when Internet companies such as Google and Facebook have wrestled with foreign governments over freedom of speech and privacy issues as they expand rapidly overseas.

In 2010 Google relocated its Web search engine to Hong Kong, following a very public spat with the Chinese government over its refusal to bow to Beijing's Web censorship requirements and a hacking episode that Google said it had traced to China.