Farnaz Seifi was arrested as she waited to fly out of a Tehran airport in 2007. Her crime? A blog calling for women's equality in the Islamic Republic of Iran.

Originally on her way to a cyber-journalism workshop in India, Seifi, then in her mid-twenties, was transferred to Tehran's notorious Evin prison and at midnight, the interrogation began. According to the authorities, says Seifi, challenging the inequality and injustice that women face in Iran was "acting against national security and being a spy for western countries."

Seifi is one of three women featured in a new documentary, "Forbidden Voices," that follows the lives of female dissident bloggers across the globe.

These women pay a high price for their blogs -- suffering violence, harassment and detainment by some of the world's most repressive regimes.

"They don't use political propaganda, they don't make speeches, they don't try to change things really aggressively -- they are trying to start a dialogue and I think this is why the world started noticing them," said Barbara Miller, director of "Forbidden Voices," who was struck by the personal nature of blogging and how it allows ordinary people to talk about the difficulties they face. "I think this is a way that women tell stories and how women try to change the world."

Friday marks World Press Freedom Day, and this year the is emphasis is on securing freedom of expression in all media, including a free and open internet -- something Seifi can identify with.

She told CNN: "In countries like my country, you learn to censor yourself from a very early age and you become good at censoring yourself, unfortunately. The Islamic Republic of Iran makes each of us an example of it because we also censor each other."

"It's sad when one day you figure out that you want freedom so badly but you are also part of the censoring system."

After a series of interrogations and heavy censorship of her blog by the Iranian authorities, Seifi left Iran in 2007 and now lives in Germany. As a women's activist and blogger means it's unlikely she will return home while the current Iranian regime is in power. And for her family's safety she now blogs anonymously.

Cuba's Yoani Sanchez, perhaps the most well-known of the three female bloggers featured in Miller's film, continues to speak out against the Castro government on her blog, "Generación Y," which receives millions of hits each month. She was finally granted a passport by Cuban authorities in January after 20 failed applications, and her recent high profile trip to the U.S., where she visited the White House, has turned the media spotlight on the daily struggle of many in communist Cuba.

But others remain trapped. Some of the film's most sinister scenes show Chinese blogger Zeng Jinyan under house arrest in Beijing, harassed by security officials outside her apartment as she tries to leave. When her husband, AIDS and environmental activist Hu Jia, is sentenced to over three years in prison in 2008, she was is left alone and isolated with their baby daughter and her blog, wondering if they will ever be free. Since then, her blog has been censored in China.

In countries where the state has a monopoly on information, blogging has come into its own as a tool for change and an outlet for sharing stories, according to Miller. "The governments really fear these women. I mean, they are women, talking about their daily lives but the governments fear that they are so outspoken, that they are talking about what's really happening in their countries".

A refusal to be censored unites these women across cultures and continents, as they use their blogs to connect with the world outside of the regime. For Miller, the reaction of the authorities is perhaps the strongest indication of the strength of their individual voices.

"I think blogging is an extremely powerful tool, you can reach the whole world with one voice."