As the resident of a quiet village in Oxfordshire with a plummy accent to match, she makes an unlikely revolutionary. But she has become a key player in the unrest that is shaking Iran and is such an irritant to President Mahmoud Ahmadinejad that she has been subjected to a propaganda campaign by the regime's henchmen.

Known only by her Twitter name, Oxfordgirl has emerged as a crucial link between the protesters and the outside world. "Before they started blocking mobile phones I was almost co-ordinating people's individual movements – 'Go to such and such street,' or 'Don't go there, the Basij [militia] are waiting,' " she said. "It was very strange to be sitting in Oxford and co-ordinating things like that."

Tomorrow the opposition is planning another demonstration under the cloak of an official rally to mark the 31st anniversary of the revolution. Oxfordgirl, who guards her identity for fear of reprisals against her family in Iran, said: "It's going to be a big day for the Persian psyche. It won't topple the regime but it's part of the process of showing the resistance won't go away.

"It's significant because of the symbolism of the revolution. A lot of people will attend the official rally and see lots of protesters coming out against the regime."

Over the last seven months Oxfordgirl has built a reputation as one of the most reliable sources of information on the turmoil. Since the disputed election last June she has posted more than 12,000 updates on Twitter, and has become convinced that the social networking site is helping to bring down the regime.

"People who haven't been involved in Iran don't understand how Twitter can work – they think it's about chatting about pop stars. But if it hadn't been for Twitter a lot of people wouldn't have got involved [in the unrest] and they wouldn't know what's going on.

"On a practical level it has saved lots of lives by warning people not to go down certain roads."

A former journalist in Tehran, she has used her contacts to spread word of the unrest. With heavy restrictions on foreign media, Twitter updates from Oxfordgirl and a handful of others counter-balance the official version of events.

"In the early days I was posting news of riots in other cities, at a time when the international media was saying it was only in Tehran," she said. "Several days later the BBC confirmed there had been riots elsewhere. This made what I was tweeting more trustworthy."

In the run-up to tomorrow's protest she has been using her 10,000 Twitter followers to disseminate ways of avoiding the anticipated crackdown. "The regime is getting better at shutting down the internet and my contacts are nervous about what might happen," she said. "But Iranians are clever at getting around things."

Oxfordgirl's effectiveness appears to have rattled the regime. She is convinced that the Iranian government has tried to use Twitter itself to undermine her.

"One day a whole load of new people arrived on Twitter. It was quite clear that some ministry got them to join at the same time and follow each other. They started putting out rumours about me. When Persiankiwi [another prominent Twitter user] went silent, they said it was me who had turned him in. Then they started saying I was Maryam Rajavi [an exiled opposition leader regarded as a terrorist in Iran]."

She is acutely aware of the dangers of being discovered. "I live in a small village so anyone who is out of place stands out immediately. There have been a couple of moments when I've seen people outside, and my heart started going faster. Your imagination runs wild. I don't want my cousins disappeared in the middle of the night."

Despite the risks she is determined to carry on. "I'm doing this because I love Iran and I want to it to be free," she said. "I don't want people to be frightened of what they say."