Criticism is escalating over censorship of online social networking sites in Korea after Twitter’s announcement that it will enable certain countries to withhold content if it goes against their law.

“Twitter has changed my life over the past year and it was an extraordinary tool that I wanted to thank,” said a Twitter user named Kim Yeo-jin on Sunday.

“It’s just sad news for me to hear that Twitter has decided to censor tweets upon specific requests from a country and as a result, I’ve decided to join the move to participate in the Twitter blackout campaign for one day.”

Kim is one of thousands of people across the world who have taken part in the Twitter blackout campaign ― posting no tweets since 4 p.m. Saturday, local time.

Earlier on Friday, Twitter posted on its official blog that it will issue an alert box that says “Twitter withheld” or “@Username withheld” in place of a Tweet or account when it is filtered by the company.

“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,” it said.

The statement indicates that Twitter will withhold content when it is asked to do so through a “valid and applicable” legal request.

In a related move, the Korea Communications Standards Commission also said it agreed to introduce a warning system for online social networking services, such as Facebook and Twitter, during a meeting earlier this month.

The commission said it will give a warning message to users who post illegal or harmful messages, requesting their deletion within a day. If not deleted, the commission will ask Internet service providers to block people from connecting to the person’s SNS account, it said.

“We have put the measure in place to review SNS content more thoroughly and to minimize the consequences that may result from excessive blocking of online posts,” said an official at the commission. “We plan to make revisions to the system if we develop ways to delete only the illegal postings.”

In response, some Web users complained their freedom on speech online was being strangled.

“I’m shocked that the government is trying to regulate the social networking sites. It’s long past the time when they attempted to block the eyes and ears of the people,” said a Twitter user surnamed Won.

By Cho Ji-hyun (sharon@heraldcorp.com)