Bloggers are being asked to show their support for freedom of expression by Amnesty International
The human rights group also wants web log writers to highlight the plight of fellow bloggers jailed for what they wrote in their online journals.
The organisation said fundamental rights such as free speech faced graver threats than ever before.
The campaign coincides with the start of a week-long UN-organised conference that will debate the future of the net.
"Freedom of expression online is a right, not a privilege - but it's a right that needs defending," said Steve Ballinger of Amnesty International. "We're asking bloggers worldwide to show their solidarity with web users in countries where they can face jail just for criticising the government."
Mr Ballinger said the case of Iranian blogger Kianoosh Sanjari was just one example of the dangers that some online writers can face. Mr Sanjari was arrested in early October following his blogging about conflicts between the Iranian police and the supporters of Shia cleric Ayatollah Boroujerdi.
Amnesty wanted bloggers to publicise cases such as this, said Mr Ballinger, and to declare their backing for the right to free speech online.
Amnesty calls on bloggers to unite
Amnesty International Statement at the opening of the Internet Governance Forum, Athens 2006
"Call to Bloggers" to stand up for freedom ahead of world meeting on future of Internet