Nasrin Sotoudeh's seven-year-old son, Nima, wants to go out to play. His mother, the leading Iranian human rights lawyer whose arbitrary imprisonment in 2010 sparked an international campaign to free her, has been talking for ages. Nima is bored.

At the door to their apartment in north-west Tehran, Nasrin takes Nima in her arms. The boy stands on tip-toe to embrace his mother. They hold each other for a minute or more. It is as though the two cannot bear to be separated.

If so, it is hardly surprising. Nima was only three when the silent men from Iran's ministry of intelligence came for his mother in 2009. Nobody knew if she was ever coming back. Her initial jail sentence was 11 years. She was held in solitary confinement, denied visits and phone calls. Her health deteriorated, she lost weight. There were rumours she had disappeared. Then, unexpectedly, Sotoudeh was released, without explanation or apology, last September.

Nima never wants to let her go again.

Speaking candidly to the Guardian despite a risk that the interview may provoke official retribution, Sotoudeh said it was Nima who was indirectly responsible for her first hunger strike, which attracted international attention to her plight.

"Before the 2009 [presidential] election I was threatened many times for my work as a human rights lawyer but there was no serious problem. But after the election [which was won by Mahmoud Ahmadinejad amid claims of fraud and widespread street violence], things changed.

"I was at a meeting of the Professional Women Lawyers Association [a pressure group she helped to found]. Suddenly the door was thrown open and some intelligence police came in. They showed me a warrant from the court and they told me to come to the court after three days. I went, and they arrested me.

"At the same time, five men went to my house and searched it. They took some personal items away. I had written a diary about Nima, every day from the day of his birth, and they took this away.

"When I was in prison I asked them to return the diary and the other personal items. For two weeks I did not even have a telephone to call my husband. So I went on hunger strike for three days. Then they gave me a telephone and returned all the items to my house."

It was a small victory, and an important one, but Sotoudeh's incarceration was only beginning. After a lengthy and often frightening interrogation, she was eventually charged with "acting against national security" and "propaganda against the regime".

Sotoudeh was sentenced to 11 years in prison, barred from practising law for 20 years, and banned from leaving Iran. An appeal court reduced the sentence to six years and her law ban to 10 years.

In May 2010, nine months after she was taken, Sotoudeh wrote to Nima from Tehran's notorious Evin prison, where she was held in Ward 209, reserved for political prisoners and run by the intelligence ministry.

Writing on tissue paper (which was all that was available to her), she tried to explain the inexplicable, and to bridge the gulf of fear and incomprehension that she worried was growing up between her and her son.

"Hello my dearest Nima," it began. "Writing a letter to you my dear Nima is so very difficult. How do I tell you where I am when you are so innocent and too young to comprehend the true meaning of words such as prison, arrest, sentence, trial, injustice, censorship, oppression versus liberation, freedom, justice, equality?"

"How do I explain that coming home is not up to me, that I am not free to rush back to you, when I know that you had told your father to ask me to finish my work so I can come back home? How do I explain that in the past six months I was not afforded the right to see you for even one hour?

"My dear Nima, in the past six months, I found myself crying uncontrollably on two occasions. The first time was when my father passed away and I was deprived of grieving and attending his funeral. The second was the day you asked me to come home and I couldn't come home with you. I returned to my cell and sobbed without control."

There were other bad moments – many of them, in truth – though Sotoudeh is a modest, self-effacing woman who is loath to dramatise her experiences.

For many Iranians, especially younger women, she is a national hero, though their praise is rendered privately, not spoken out loud. Despite winning the European parliament's 2012 Sakharov prize, and several other awards, Sotoudeh says she always feels surprised when people tell her she is famous.

Aged 50, slim and of average height, with short brown hair and a brilliant smile, Sotoudeh seems happy in her home. But smudges under her eyes and a certain nervous edginess betray a different, darker reality. Her eyesight suffered in jail, she said, but she was otherwise in good health. Given the awful things that happened to her, she appeared remarkably sanguine.

"I started my job as a lawyer after qualifying – it took me eight years to get my certificate – by taking on human rights cases. I specialised in the rights of children. I was very sensitive about capital punishment, especially of under-18s, and this is still a problem in Iran. I was interested in the rights of women, of political activists, of journalists and religious minorities. I spent 10 years working on these cases."

Apart from running her own law practice, Nasrin said she worked with or helped create NGOs including the Defenders of Human Rights Centre, which was founded by Shirin Ebadi, the 2003 Nobel peace prize-winner, and the Children's Rights Committee. The human rights centre was shut down by the government in 2008.

Among those she represented before her arrest were Isa Saharkhiz, a pro-reform journalist, Heshmat Tabarzadi, an opposition political activist, and Parvin Ardalan, who founded the One Million Signatures campaign for equal rights for women and won the 2008 Olof Palme prize.

"I was also involved with the case of a 19-year-old, Arash Rahmanipour, who was arrested before the [2009] election but who they [government-controlled media outlets] said was arrested afterwards, during the fighting. It was a lie. They got very mad at me. [Rahmanipour was convicted of trying to overthrow the regime and "waging war against God" and was hanged in January 2010]

"They [the authorities] did not like me defending such cases. Many times they asked me to leave this job. They also did not like what I was saying in interviews with domestic and international media. When they arrested me, they said they had made a CD of all my interviews. I said I had not done anything wrong."

Her interrogation and trial, before a special court, took place in camera at Evin prison, she said. "This was the very worst place. The interrogations were done by representatives of the intelligence ministry. They were really tough and bad and scary. I demanded my freedom. What did they do? They added a third charge – being a member of the Defenders of Human Rights Centre, which carries a term of five years.

"I had been in solitary confinement up until now. There was no communication. There were five cells on my corridor but it was all about silence and solitariness … The worst moment was one day when I was locked in a room alone with a male interrogator. This is not normal, to be alone with the interrogator. I remember there was only a small window. I screamed.

"Then the deputy of the jail came with a woman. He ordered that the woman must be present at all my interrogations. If you ask me, I would say physically I was not mistreated, but I was threatened a lot psychologically.

"My interrogator said: 'I am not going to release you, I will get you a sentence of 10 years.' In fact I got 11 years. He was behaving like he had the power to make the judges do what he said. And they did."

Later on in her jail term, Sotoudeh said, her chief interrogator suggested she give media interviews that favoured the government, as a means of mitigating her "guilt". "I was laughing at him. I said: 'Is that what you think of me?'"

This suggestion, smacking of desperation, may have been prompted by the gathering international campaign to free Sotoudeh, her nomination by Amnesty International as a prisoner of conscience, and statements of concern about her treatment from the US and other governments.

Human Rights Watch, the International Commission of Jurists, the International Federation for Human Rights and the European parliament all weighed in on her behalf. So, too, did some western media outlets and the Law Society of England and Wales, which called for her release in January, 2011.

A few months later, said Sotoudeh, the same interrogator returned. "He said: 'OK, don't give interviews. Just go back to your lawyers' meetings and be our informer.' And I laughed at him again. He came so many times, threatening me. He said: 'I am going to break you down. I will rub your nose in the ground. I will keep you here for ever.'"

Sotoudeh said being separated from her children – Nima, and her daughter, Mehraveh, who was 11 when she was arrested – was perhaps the hardest thing to bear. But her children, young as they were, had shown great courage.

In October 2012, she launched a second hunger strike, which lasted 49 days, to protest at her prison conditions, including restrictions on family visits, and a travel ban imposed on her husband, Reza Khandan, and on Mehraveh.

During this period, at a rare meeting with Mehraveh from behind a glass screen, Sotoudeh said the prison authorities distributed what was called a forgiveness paper, which allowed prisoners to claim a furlough in return for admitting guilt.

"On that day my daughter was very anxious, so they let her come close to me. I hugged her. I could feel she was very concerned about me. I told her they were distributing the forgiveness paper. I said: 'I can take one.' But my daughter replied: 'Don't even think about it.'"

At this time, Khandan expressed fears for his wife's life, saying she was experiencing dizziness, impaired vision, low blood pressure and drastic weight loss. Eventually, her demands were met, the travel ban was rescinded, and she ended the hunger strike.

Had she ever thought of giving up, or giving in? Yes, she replied, she had had her share of dark moments.

"We are all human beings. Sometimes you feel weak and devilish thinking comes to us from Satan. But I can tell you when it came to doing interviews for them or cooperating with them, I never had a second thought."

Sotoudeh said her unconditional release in September last year came as a surprise. It coincided with the coming to power of a new, less hard-line Iranian president, the centrist Hassan Rouhani, and a high-profile trip he was making to the UN general assembly in New York. Twenty other political prisoners were also freed, she said. No explanation was given.

"Unfortunately, since then this process has stopped," Sotoudeh said, noting that up to 800 political prisoners remain in custody. "I have a very bad feeling about the other women prisoners that I left behind. We lived together for three years – some minorities, Baha'is, Christians, political activists from the Greens [opposition reformists], some leftwingers such as communists, also journalists and writers.

"I know them. I was released without any request being made. We expected that they would be released too, but this has not happened … Still I am hopeful the government could do it.

"At one time there were 30 women in prison, now there are 14. Some have been pardoned, some have completed their terms. The good news is that they have not taken any more prisoners and the total is going down."

Home again, with the judicial slate apparently wiped clean, and despite or perhaps because of her three-year ordeal, Sotoudeh has resumed her work as a human rights lawyer. She is defending two cases, both concerning juvenile offenders under the age of 18 who are charged with capital crimes, and she is considering a third, a political case.

She says she has also reactivated her Professional Women Lawyers Association and Children's Rights Committee, and has launched a campaign with seven others for the gradual abolition of capital punishment in Iran."Human rights campaigners need to concentrate on two things," she said. "First, reduce capital punishment, which has been increasing under the new government, and second, press Mr Rouhani to react to actions that are against human rights in Iran like the attack on prisoners in Evin jail three weeks ago. If Mr Rouhani cannot react properly to such crises, soon he will lose his support in society."

Is she frightened that her return to the human rights battlefield may have new, negative consequences for herself and her family? Sotoudeh says she is not concerned. "Since I was released they haven't bothered me, they haven't contacted me."

All the same, it may be no coincidence that her apartment was broken into four months ago, shortly after her release. And her husband recently received an anonymous threat from somebody threatening to throw acid in his face.

Nasrin smiles; Nima has come back in, now the talking is over. He wants to play a tune on the piano for his mother and her guest. He plays, we listen. And hope for the best.