Thousands of people are taking part in "virtual protests" against countries accused of censoring the internet.
One of the most targeted sites is the video-sharing website, YouTube
For its first Online Free Expression Day, media watchdog Reporters Without Borders (RSF) has created virtual versions of nine public spaces.
These areas where protests are not normally possible include Beijing's Tiananmen Square and Kim Il-Sung Square in the North Korean capital Pyongyang.
RSF says at least 62 cyber-dissidents are in jail around the world.
More than 2,600 websites, blogs, or forums were closed down or blocked last year, the group says.
The RSF demonstrations are running for 24 hours, until 1000 GMT on 13 March. Several hours after they opened, more than 5,000 virtual dissidents were protesting online.
Users are invited to create an avatar, choose a message for a banner and take part in one of the demonstrations in Burma, China, Cuba, Egypt, Eritrea, North Korea, Tunisia, Turkmenistan and Vietnam.
This year, RSF has added Zimbabwe and Ethiopia to its list of "internet enemies".
The other countries blacklisted by the group are Belarus, Burma, China, Cuba, Egypt, Iran, North Korea, Saudi Arabia, Syria, Tunisia, Turkmenistan, Uzbekistan, and Vietnam.
"From now on, we will organise activities every 12 March to condemn cyber-censorship throughout the world," RSF said in a statement.
"A response of this kind is needed to the growing tendency to crack down on bloggers and to close websites."
One of the most targeted internet sites is the popular video-sharing website, YouTube, that has been blocked or banned in a number of countries including China, Iran, Pakistan, Turkey, Burma and Brazil.
The reasons for banning the site vary from country to country, but include government objections to religious and political material, foreign sites and pornography.