Anders Behring Breivik has told a court that his "primary target" in last year's terrorist attacks was a former prime minister whom he planned to behead, posting the footage on the internet – and that he anticipated all 564 people on Utøya would die in his "operation".

Giving evidence on the fourth day of his trial, the 33-year-old said he would have preferred to carry out three bomb attacks rather than target Utøya, where the Norwegian Labour party was holding its annual youth summer camp on 22 July last year. In the end, he went on the rampage on the island after planting one bomb in Oslo's government district, killing eight people.

Breivik claimed he was "forced" to carry out the massacre on the island, which left 69 dead, because Norwegian and EU regulations had made it difficult to acquire sufficient bomb-making equipment.

Bombing was much easier on the emotions than pulling a gun trigger, he said. "It's easy to press a button and detonate a bomb. It's very, very difficult to carry out something as barbaric as a firearm-based action."

To do so, he claimed, was not natural. "It is contrary to human nature to execute something like this," he said. "You have to work on yourself for a very long time to make yourself do this ... to hammer away at your emotions."

His original plan for the attack on Utøya was to time his arrival on the island with a visit from Gro Harlem Brundtland, a former Labour prime minister of Norway. Breivik told the court he planned to handcuff her, before "decapitating" her using a bayonet on his rifle and then filming the execution on an iPhone.

"The plan was to chop her head off with [the bayonet] while reading a text and then upload the film to the internet," he said.

Brundtland was his main target, said Breivik, adding that he nonetheless expected everyone else on the island to die. "The objective was not to kill 69 people on Utøya. The objective was to kill all of them," he said, explaining that he planned to scare the campers into the water.

"The main goal was to use the water as a mass destruction method. Basically, I assumed most people would drown," adding that "it's hard to swim if you have death anxiety".

Brundtland survived, having left the island by the time Breivik arrived.

Breivik insisted those he killed on Utøya, some as young as 14, were "legitimate targets". He said: "I am not a child murderer. I believe that all political activists who choose to fight for multiculturalism ... and have leadership positions are legitimate targets."

He admitted he would rather have bombed the parliament or the Labour party congress, but that it took him far longer than he anticipated to make the bomb, and by the time he was finished, parliament was in recess and most politicians were on holiday.

Questioned about why he chose to bomb Oslo's government district, Breivik revealed his original plan was to also bomb two other targets in the Norwegian capital: the Labour party's office and a third target, possibly the royal palace, the parliament or the headquarters of the newspapers Aftenposten and Dagsavisen.

"I settled on the palace in a setting where the royal family wouldn't be hurt," he said. "Most nationalists and cultural conservatives are supporters of the monarchy, including myself."

The anti-Muslim fanatic said the three bombs would be followed by several shooting massacres, if he survived. He decided against multiple bombs because building one was "much more difficult than I thought".

Yet Breivik also said that "if it hadn't been for the EU and Norway's rules on explosives ... there would have been three car bombs". It was very difficult to source the sufficient amount of fertiliser required to make a bomb, he claimed. In order to acquire the necessary amount, he had to rent a farm with the requisite area of land so that the fertiliser companies would not get suspicious and "flag" him to the security services.

It was from a farm about 140km north-east of Oslo, that he made the bomb, which killed eight in the capital's government district.

He claimed he had "trained" for the "operation" by going on long walks wearing rucksacks filled with 30kg of rocks, aiming to mimic the weight of body armour. He also carried out "reconnaissance missions"on potential targets by walking past them as unobtrusively as possible.

Earlier on Thursday Breivik revealed that hehad practised shooting by playing the computer game Call of Duty: Modern Warfare.

The 33-year-old said he practised his shot using a "holographic aiming device" on the war simulation game, which he said is used by armies around the world for training. He used a similar device — a gun attachment allowing the user to look through a glass optical window and see their target image superimposed at a distance -in the "real world" during the shooting attacks. In addition, Breivik trained 25 times at Oslo Pistol Club in order to be allowed to legally own the guns he used on 22 July.

The court also heard Breivik took what he called a sabbatical for a year between the summers of 2006 and 2007, which he devoted to playing another game, World of Warcraft (WoW), "hardcore" full time.

But he insisted WoW had nothing to do with the attacks. He said: "Some people like to play golf, some like to sail, I played WoW. It had nothing to do with 22 July. It's not a world you are engulfed by. It's simply a hobby."