Security measures at Imam Khomeini international airport have been doubled. I was pulled aside by a burly guard who asked to look at my passport and asked a list of questions. Where was I travelling from? Why was I coming to Iran? Did I have any alcohol or drugs in my luggage? He stared at my passport photo, then at me and back again, narrowing his eyes. Had I been too hasty in laughing off warnings that my Facebook and Twitter activism back in the UK would land me in trouble?

I was eventually given back my passport and luggage and sent on my way. It's an hour's drive to the city from the airport, and it didn't take long for the conversation with my taxi driver to turn political. "I voted for Ahmadinejad, and I'd vote for him again if they hold another election," he told me.

"Everyone goes on about it being impossible for Ahmadinejad to win by such a large margin, but that's because they only see their own friends, their own families, their own social circles, which are all the same.

"The people who voted for Ahmadinejad are those who don't have a voice. The poor, the less educated, people who don't live in Tehran, don't drive fancy cars, have to work 16-hour days just so their children won't go hungry. The rest of the country have more important things to worry about, like how to put food on the table, not falling behind on the rent, saving money to send their children to university.Ahmadinejad has done so much to help those people in the past four years, and they have now repaid him with their votes." Despite what he saysabout the extent of Ahmadinejad's support, the atmosphere in Tehran is thick with fear. We drive past Basij militiamen stationed in the main streets, holding rifles and stop-searching cars at random. Even my pro-Ahmadinejad driver warns me not to look at them for too long.

I began to call friends as soon as I got home. No one was prepared to talk over the phone, all convinced their calls were being monitored. Few were prepared to meet. "It's too dangerous, the city is in the hands of the Basij," one friend whispered. "They're everywhere and they're listening. I know a few people who might be prepared to talk to you, but please be careful. And whatever you do, stay away from Baharestan Square. They're shooting people."

Half an hour later, I got off the bus one stop away from Baharestan, where the Iranian parliament building is located. I could see the huge pyramid shape over the rooftops. My heart sank as I started walking towards it. Three rows of riot police were standing in the middle of the street, with guards patrolling the pavements on either side. A passerby told me it was the same for all the streets leading to the square.

I'll be the first to admit the plan I came up with was pretty awful. I ran up to the first row of police and begged them to let me pass. It's an emergency, I told them. I lived just off the square with my elderly mother and she'd just called me because she was scared – I needed to get to her.If they just let me pass through, I wouldn't cause any trouble.

The policeman who'd turned round to listen shook his head and got back into position. "Please!" I wailed, grabbing his arm. He knocked me back so hard I fell on the ground. "Get out of here," he growled.

I walked off until the guards were out of sight and slipped into an alley. I was halfway down the narrow street when I heard heavy footsteps behind me. I broke into a run, flew round a corner and ducked behind a bin, my heart racing. The footsteps came closer, then suddenly stopped. A policeman towered over me. He raised the visor on his helmet, grabbed my arms and pulled me up so roughly I cried out in pain.

"I told you to get out of here," he said angrily. But after a moment he sighed, "Go away and stay away. We have the blood of our countrymen on our batons. I don't want any more on mine." He let go of me and I ran. Later, in a dingy coffee shop in Enghelab Street, I met up with a prominent activist in the recent demonstrations . He spoke in a voice so low I could barely hear him.

"It's true things have quietened down now, but that's not because people no longer care. Everyone's scared.

"All we're asking for is for another election to be held. We're not questioning the Islamic Republic. [Ayatollah Ali] Khamenei was our last hope, but his sermon [last Friday] made us out to be the enemies of Islam and Iran. But we're not. We need to make the government understand we're all on the same side. We need to carry on protesting but work towards understanding at the same time, or we're all doomed." I've always thought that Iran is a country where if you demonstrate for freedom of speech, you may have a million or two behind you. If you organise a mourning demonstration for a Shia imam, you'll have 30 million behind you. Religion runs deep here, and for many, religion has manifested itself in Khamenei, the country's leader. The recent unrest, however, shows that the voice of dissent is growing. The call for change is louder than ever before and the clerics can't afford to ignore it. The streets are quiet for now, but if you go to the rooftop at 10pm every night you can hear people shouting "Allahu Akbar" (God is greatest) in the darkness.

Although the opposition movement is without a definite leader or a definite goal, anger and resentment are bubbling away under the surface and will reach boiling point very soon. The election – rigged or not – was the straw that broke the camel's back for Iranians who want more freedom. Once the anger bubbles over and the people take to the streets again, all it needs is for a hardline cleric to give a fatwa for jihad against Mir Hossein Mousavi's supporters and the country will descend into blood and chaos. Khamenei must do something, and fast.

Sogol Baharan is a pseudonym