The father of the former KGB officer Alexander Litvinenko has accused President Vladimir Putin of ordering his murder, claiming that no one else in Russia would have the authority to sanction an assassination on foreign soil.

In his first interview since his son’s death, Walter Litvinenko, who served as a doctor in the Gulag during the Communist years, said he was convinced that Alexander was poisoned by the FSB – the successor to the KGB. “The cynical murder of my son was a calculated act of intimidation,” he said. “I have no doubt that he was killed by the FSB, and that the order came from that former KGB spy President Putin. He was the only person who could give that order. I haven’t a shadow of a doubt that this was done by Putin’s men.”

The comments will infuriate the Kremlin, which is still trying to ride out the political storm that followed Mr Litvinenko’s death on 23 November, after being poisoned with the radioactive element polonium-210 in London on 1 November. Mr Litvinenko also accused Russia’s President of running an “authoritarian” regime, and claimed: “Bush and Blair have trusted him too much. They shouldn’t have trusted him.”

In Moscow today, thousands of Russians are expected to join a march to protest against Mr Putin’s increasing authoritarianism. The officially banned protest, called the “March of Those Who Don’t Agree”, is being organised by a coalition of anti-Kremlin forces that includes liberal free marketeers, nationalists and radical youth movements. Organisers include the former world chess champion Garry Kasparov, the former prime minister Mikhail Kasyanov, and Eduard Limonov, a radical nationalist whose National Bolshevik party is officially banned.

Mr Litvinenko also described how his son startled him by revealing that he had made a deathbed conversion to Islam. All four of his children were raised in the Russian Orthodox Church. But Nalchik, near Chechnya, where Alexander grew up, is in a region where Muslims outnumber Christians. Mr Litvinenko, who speaks no English, also said that he would go back to Russia, despite his suspicions about Mr Putin. Asked whether he might be putting himself at risk if The Independent reported his accusations, he said: “Of course it will be dangerous, but you must write it anyway. If you don’t write it, then I have betrayed my son.”

He said that he was in Russia when he first heard that his son was in hospital. He prayed intently, but soon afterwards he learnt that Alexander – known in the family as Sasha – wanted to see him urgently. The train journey to Moscow took two days. Mr Litvinenko was so sick with anxiety that he could not eat until he was on the plane to London.

When he arrived at the hospital, he found his son close to death. “I tried to look at his face, and it was full of suffering. It was always a joy to look at my son, because he was so handsome, but now it was difficult to recognise him. His eyes were closed. I made the sign of the cross over him, and read the Paternoster. I said, ‘Sasha, I saw the light when I was praying to Saint Sergei, and I know you will get better.’

“I told him a lie. I said, ‘Sasha, the poison inside you has gone. All you have got to do now is overcome the effects of the poison.’ Sasha opened his eyes. He sat up, exactly as in the photograph in all the newspapers. And I saw huge hope in his eyes. He said: ‘Papa, I have something serious to tell you.’ His eyes were very, very serious: I’ll always remember that look. He told me: ‘I converted to Islam.’

“I thought that this was very strange. It was as if our roles had been exchanged; it was as if he had become my father, and I had become his son. I said, ‘Sasha – anything. Anything, as long as you don’t turn into a Communist or worship the devil.’

“I could only be with him for two evenings. He suffered in front of me. Something hugely powerful was acting inside him, and it was killing him. He was being torn apart from inside. Sasha told me: ‘The long arm of Putin reached out and got me here.’ He was very strong, and very courageous, as he struggled to live. Books will be written about him.”

Mr Litvinenko broke his silence as the investigation into his son’s death intensified. The third man who met Alexander at the Millennium Hotel on 1 November, Vyacheslav Sokolenko, was interviewed yesterday by Russian officials and British police. A team of Scotland Yard officers in Moscow has already met the other two men, Dmitry Kovtun and Andrei Lugovoi – who yesterday protested his innocence.

Mr Litvinenko, 68, had his own clash with authority in 1979, when he was working with mentally disturbed prisoners in Sakhalin island, in Russia’s far east, with the rank of captain in the USSR’s Ministry of Interior troops. He said he resigned his commission after an official investigation failed to back his complaints about the treatment of detainees. He retrained as a psychiatrist.

Alexander’s grandfather was a fighter pilot. One day the boy made the mistake of saying that he did not want to grow up to be a “bloody soldier”. His furious grandfather cracked a plate over his head. Alexander enlisted at the age of 17 in the Dzerzhinsky Division and was offered a transfer to the KGB in 1988.

“One day, Sasha called me and said, ‘Listen Dad, I have a suggestion from the KGB that I should join them. What would you say?’ At that time, the KGB had a legendary aura, and I said, ‘Yes, son – go ahead and serve your motherland.’

“He did not talk much about his time with the KGB. What he did say was that he used to infiltrate gangs. On the internet there are reports saying that Sasha used to be a spy. That’s simply a lie. He was mostly in counter-intelligence.”

In 1998, Alexander gave a now famous press conference, alleging that the FSB was forming a hit squad to eliminate political troublemakers, and he had himself been commissioned to kill Boris Berezovsky. He was arrested in 1999.

Mr Litvinenko said: “They lodged six charges against him, but four were dropped. It was obvious that these charges were fabricated. They found a couple of quite ridiculous things. He was accused of pilfering some cans of food, and of slapping someone in the face.

“Then they produced a fake video in which a blond man wearing camouflage was beating up a detainee. But thankfully, Alexander’s friends knew about this video and found the original, in which Sasha wasn’t beating up the detainee. That’s when the FSB came to him and said: ‘You have a son. If you produce that video in court, you should be very afraid for your son.’ That was one of the reasons why he decided to run.

“It wasn’t just coincidence that Sasha came to London, and it wasn’t coincidence that he started to fight against one of the most terroristic organisations in the world, which is headed by President Putin. Sasha never betrayed anyone and I say to the people for whom he gave his life ‘Please don’t betray Sasha’.”