Anders Behring Breivik produced "cries of joy" during the massacre he carried out on the Norwegian island of Utoeya last July, in which 69 people died, his trial has heard.
In the first of several witness accounts on Wednesday, Tonje Brenna said she heard Breivik whoop with joy as he shot people on the island.
Another witness said he saw Breivik fire at those trying to swim to safety.
Breivik admits the murders, but denies criminal responsibility.
He killed 77 people in total - 69 at a youth camp on Utoeya and eight in a bomb attack in the capital Oslo earlier on 22 July 2011.
Breivik says he was fighting to defend Norway from multiculturalism and immigration.
He claims that the Labour Party youth meeting at which he committed the massacre was a "legitimate target" because of its support for multiculturalism.
Testifying, the leader of the Labour Party's youth wing, Tonje Brenna, 24, who organised the event on Utoeya, told the Oslo court how she heard what she took to be firecrackers while organising a meeting about the Oslo attack earlier in the day.
"I told somebody on the phone: 'Someone has a bad sense of humour, they're fooling around,'" she told the court.
Running towards the source of the noise, however, she saw two or three people hit ground and realised they were being shot.
Ms Brenna said scenes of panic ensued. She described hiding from Breivik on a rock ledge by the lakeside, together with a girl who was bleeding profusely.
She said she could hear Breivik fire down on them and people falling on to the rocks around them.
"We told ourselves that tomorrow we will be at home, we can watch the Saturday films with our parents and eat popcorn. But there was a feeling of complete abandonment and of hopelessness - nobody could do anything.
"We were cold and wet and covered in blood. I thought it was just a question of time before I was hit."
Breivik smiled and shook his head in court when Ms Brenna described hearing him emit sounds of joy, the BBC's Steve Rosenberg reports from the hearing in Oslo.
"I'm sure I heard cries of joy," Ms Brenna told the court. "If I had to spell it out, it would be woo-hoo."
Giving evidence last month, Breivik insisted he neither smiled nor laughed during the attack.
The killer later expressed frustration when told by the judge that he could not question the witnesses directly.
Another witness, local boat owner Oddvar Hansen, described how he had rushed to Utoeya after hearing of the situation there.
After pulling two girls aboard, Mr Hansen saw people in the water crying and waving for his help off the southern tip of Utoeya, where Breivik carried out the last of the killings.
But Mr Hansen said he had made the "tough choice" of deciding to help those further from the shore. "It was hard to live with that decision afterwards."
Breivik has said his plan was to kill as many people as possible by scaring them into drowning in the lake.
A third witness, Bjoern Ihler, described how he and two boys saved themselves by leaping into the lake when Breivik, clad in a fake policeman's uniform, calmly called out to them before opening fire.
The trial's outcome hinges on whether the court finds Breivik to have been sane or not, as it could determine whether he is sent to prison or to a psychiatric institution if found guilty.
An initial psychiatrists' report on Breivik declared him insane and not legally responsible, but a second evaluation came to the opposite conclusion.
Breivik seeks to prove he was sane, in order to prove his actions were motivated by a political doctrine.