The bestselling Swedish crime writer Henning Mankell today accused Israel of murder, piracy and kidnapping after describing how the aid ship he was travelling on was seized by Israeli forces this week.

Mankell, whose detective novels featuring the commissar Kurt Wallander have sold almost 30m copies worldwide, was aboard the Swedish ship Sofia, one of six ships in the flotilla carrying aid to Gaza. The 25-strong crew, including Mankell, were all arrested and held in custody.

In an exclusive interview with the Guardian in Berlin, Mankell, 62, described the "horrifying moment" when he realised the Israelis had chosen to attack the ships "deep in international waters".

"Our idea had been a non-violent, non-fighting back method. But we soon realised the Israelis had chosen the real, real ugly solution to attack in international water … It was only when I got on my flight home that I realised that people had died in the attack, when the stewardess told me on the plane," he said.

He called on the international community to step up its pressure on Israel to end its blockade of Gaza and said he would like to see an investigation into whether Israel could be prosecuted for its alleged attack on the ships, believing it had set out to deliberately kill protesters. Nine Turkish citizens, one of whom had a US passport, were killed in the attack.

"I think the Israeli military went out to commit murder," Mankell said. "If they had wanted to stop us they could have attacked our rudder and propeller, instead they preferred to send masked commando soldiers to attack us. This was Israel's choice to do this.

"And it was the most stupid thing they could have done, because look around, Israel has never been so criticised in the world as of today, and if you ask me, this blockade will be over within the next six months."

Mankell described being woken in the early hours of Monday morning with the news that Israeli troops were attacking the main protest ship, the Mavi Marmara, and an hour later abseiled from helicopters on to the deck of the Sofia, which was around a kilometre behind.

"We saw these black rubber boats coming with masked commando soldiers … they climbed aboard. They were very aggressive … there was an older man in the crew, he was perhaps a little slow and they shot him in the arm with an electric gun which is very, very painful … they shot another man with rubber bullets."

The soldiers checked the boat and one soon returned saying they had found weapons, Mankell said.

"I have 24 witnesses to this, he showed me my razor, a one-time use razor, and a box cutter he'd found in the kitchen," Mankell said. He said all his possessions were taken. "They stole my camera, my telephone … even my socks."

Asked if he had been naive to take part, Mankell said: "If you're saying was I a 'useful idiot', no, I don't believe I was.

"We knew from the beginning that probably we would not get our stuff into Gaza but we could anyhow win … We could get the focus on the situation. Which we did, so of course we won." He added: "Some people have asked me how did we know that the items we were due to deliver would not just have been seized by Hamas. Of course, I can't guarantee that, but when the house is burning you have to get the water." The Sofia, he said, was carrying supplies of concrete and prefabricated buildings.

Mankell said he would take part in the campaign again. He said: "Of course – in a year's time there is a plan to return. And then there could be hundreds of boats. And what will the Israelis do then? Release a nuclear bomb to stop us?"

Mankell, who has been politically active from a young age and was once a merchant seaman, said he had been struck by the lack of other writers and intellectuals on the voyage and called on others to become involved.

One moment of relief in the ordeal, he said, had been when his interrogator recognised the author, who is one of the best-selling crime writers in Hebrew.

"He said: 'You're charged with entering Israel illegally.' I said: 'That's absurd, you brought me here.' Then he said he knew who I was and that he'd read all my books and liked them. I told him: 'Next time you're in Europe call me and we'll talk about all of this.' I gave him my number – well we'll see. I do believe dialogue is the best way."

Mankell, who is married to Eva Bergman, a theatre director and daughter of the late film director Ingmar Bergman, said his motivation for getting involved had been his need to take action.

"The Gaza Strip has been transformed into the biggest open prison in the world and it was obvious we had to do something. We thought maybe we should try to break that whole blockade and the only way to do it is with a convoy of ships. When I first heard about it I thought it's a good idea, I'd like to be on board. I believe so strongly in solidarity as an instrument to change the world, and I believe in dialogue, but it's the action that proves the word."

Mankell is on a tour to promote his final Wallander novel - Stranger in the Shadows, in the German edition; The Troubled Man in the UK edition - which the disgruntled police commissar, who is suffering from the onset of Alzheimer's, sets out to solve a Cold War era crime. He said he would watch the progress of the Irish campaign ship, the Rachel Corrie, which is heading to Gaza. "We can be sure they won't attack it by sending soldiers on board, and killing people, because that would be suicidal for Israel." he said. "Maybe this time they'll attack the rudder and the propeller, we'll see."