Members of Iran's intelligence services have beaten up and temporarily detained a prominent journalist and film-maker critical of the authorities' treatment of the opposition.

Mohammad Nurizad, 61, was protesting in front of the Iranian intelligence ministry in Tehran on Monday when a group of its agents attacked him, and took him to Evin prison without a valid warrant for his arrest. He was later released.

"They pushed me on the ground as they put their knees on my back, my head hit the asphalt and blood gushed out from my eyebrows," he told the Guardian by phone from Tehran. "They didn't have a valid arrest warrant so I refused to get in their car voluntarily. They handcuffed me and forced me into the vehicle."

Every morning for the past 43 days, Nurizad has gone in front of the ministry's building wearing a white shirt and carrying a white flag with his demands written on it. "The authorities have confiscated my personal computer and belongings and have so far refused to release them," he said. "They have also imposed a travel ban on me and my family, barring us from leaving the country. I simply want these restrictions to be lifted."

Nurizad was a staunch supporter of the Iranian supreme leader, Ayatollah Ali Khamenei, and a published journalist in the ultra conservative state newspaper, Keyhan. But the events during the aftermath of the disputed 2009 presidential elections, in which tens of protesters were killed and hundreds of others arrested in the biggest unrest since the 1979 Islamic revolution, gave the veteran journalist a change of heart.

As a result, Nurizad started a campaign of writing letters to his erstwhile patron, Khamenei, accusing him of involvement in the bloody crackdown in 2009 and asking him to apologise to the nation. He was subsequently arrested and sentenced to three and half years in jail on charges of insulting officials and propaganda against the ruling system. He was released after months of solitary confinement, physical abuse and mistreatment. While in jail, Nurizad had a gastrointestinal haemorrhage owing to repeated hunger strikes.

"As commander-in-chief of the armed forces, you didn't treat people well after the election. Your agents opened fire, killed people, beat them, destroyed andburned their property. Your role in this can't be ignored ... Your apology can cool down the wrath of the people," he wrote in one of his first letters.

He said of his experience of being in an Iranian jail: "It opened my eyes. In prison, I left behind years of ignorance and naivety and came out as a new person, with open eyes."

Since his release, Nurizad has continued writing letters despite death threats and fought for human rights. In a recent move, he visited the four-year-old son of a Bahá'í family, whose parents are jailed because of their religious beliefs, which are banned in the Islamic republic. Bahá'ís are the most persecuted religious minority in Iran and have been deprived of their basic rights, such as access to education or the right to own a business. Seven leaders of the Bahá'í faith have been in prison for the past five years, each serving 20-year sentences.

Unlike other Iranian prisoners, Nurizad's popularity among the country's conservatives means he has relative protection and the authorities remain reluctant to silence him as they have other outspoken critics.

"I'm banned from working but nothing has been mentioned in my sentence. Some political prisoners in Iran are banned from working, but at least it's mentioned in their sentence. But for me, nothing is mentioned. No one dares to publish my work."

Despite this, Nurizad said he has nothing to regret. "I realised I couldn't continue with that ignorance I had some years ago," he said. "I couldn't deceive myself anymore. I wanted to get rid of the mask I was wearing."