Tehran resident Akbar described the atmosphere in an interview for the BBC shortly after returning from Wednesday's rally.

"Most of the faces were very, very young, between 17 and 27. These are soft voices, they have no defence whatsoever. If the police attacks them, they are defenceless. Currently there are no problems, but we fear that they might be attacked from the side streets. They look like a vulnerable and peaceful but well organised crowd."

There have also been reports of violence. A female student from the northern city of Mazandaran told the BBC Persian service on air that a group of students on their way home from university were stopped by guards, who offered to escort them.

"They put the boys in a car and drove them away. They took them to a police station, took their fingerprints and beat them to a pulp.

"We take to the streets to protest, but they beat us up and we are afraid for our lives. It's like martial law", she said.

Shickarchy sent this photo of Wednesday's protest in Tehran

Writing to the BBC Persian service from a small town in Isfahan province, Mahmoud made the point that it was not just the people in Tehran who were protesting.

"Guards and agents are everywhere. They may want to project an image that only people in the capital protest against the election results, but that's a lie. In my town, five members of my family put their votes for Karroubi in one ballot box, but the officials announced that Karroubi only had two votes in that box. We want the truth to be told."

Overnight, university dormitories in several Iranian cities were reportedly raided. A computer engineering student sent an email to the BBC News website describing the raid at Shiraz university.

"We were at the library preparing to start a protest. There were about 100 of us. Suddenly anti-riot police surrounded the library. They attacked us using tear gas That was about 7pm", he wrote.

For the first time since I was born I am proud to be Iranian - seeing how rich and poor, religious and agnostic get together and became one voice Tehran resident

"They took our phones and they got us to fill in questionnaires with personal details. One of my friends had a semi-professional camera and he took some photos just before the attack. He managed to avoid being caught.

"After the attack they asked us to surrender the camera threatening that they'll search all our belongings if we don't. They could not find the camera because it was not there."

Mustafa from Mashhad described what happened in his city: "Yesterday evening some two or three thousand people were holding a demonstration. The police wanted to arrest them, so many of the protesters barricaded themselves in the university of Mashhad. The police couldn't get in and those people had to stay there all night long."

Everything blocked

The Iranian government is continuing its efforts to close down any media communication beyond its control.

Wednesday's protest in the capital drew tens of thousands

Emails are monitored, phone lines are often not working and mobile phone networks are down. A Tehran resident expressed his outrage in an email to the BBC Persian service.

"Everything is blocked. We can't watch Euro news, BBC, CNN and all the rest. This is dictatorship in the true sense of the word," he wrote.

"But we will win. For the first time since I was born I am proud to be Iranian - seeing how rich and poor, religious and agnostic get together and became one voice."

A Mousavi supporter in Zahedan, who preferred to remain anonymous, also wrote with anger about not being able to watch the BBC Persian service.

"People are protesting here too. I beg you, please take Iran to court for jamming the BBC Persian's satellite signal. And thank you for your news and for reflecting the truth."

Twitter users are becoming increasingly wary about the secret services monitoring Twitter. There have been appeals to international media as well as to other Twitter users to be careful and not disclose usernames or the identity of Iranian bloggers.

A guide on how people should conduct themselves on Twitter has been circulated on the internet.

It advises Twitterers outside the capital to change their location setting to Tehran, so that it becomes more difficult to trace individuals.

It also warns that the Iranian security forces are setting up Twitter accounts to spread disinformation by posing as Iranian protesters.

There are some allegations that right-wing Israeli interests are engaged in a Twitter attack with the aim of causing political instability within Iran.