Nurpashi Kulayev, 25, a Chechen, has been declared guilty of terrorism and murder by the judge, Tamerlan Aguzarov, in the southern town of Vladikavkaz.
He will not be sentenced until after Aguzarov has finished reading his verdict, which is expected to last until at least the end of the week and involves recapping testimony from the massacre at School Number 1 in September 2004.
On Thursday, Aguzarov recalled witness testimony on how the school was stormed, with about 20 survivors and relatives of the 331 people, mainly children, who died present in court.
"Armed men in combat uniforms arrived in the school yard during a celebration for the first day of school on the morning of September 1," he told the court.
The gunmen, shooting in the air, dragged more than a thousand people, mainly children, teachers and relatives, into the school gym where they set up explosive devices and, to keep people quiet, "they pointed weapons at their children".
"The terrorists demanded a pull-out of federal troops from Chechnya. They threatened to blow up the school if security forces tried to storm it."
According to witness testimony, the bloodbath that ensued on the third day of the siege was triggered when one of the bombs laid by the assailants exploded, the judge said.
"It's not negligence, but murder"
Voice of Beslan support group
"People started to flee the hall after that explosion," Aguzarov said, recalling how the hostage-takers fired on the civilians trying to escape.
A rescue attempt by Russian commandos turned into a pitched battle and much of the school caught fire, causing massive casualties.
Many survivors and relatives are angry at the response of the authorities.
"Federal forces fired on the school with tanks and grenade-launchers, knowing there were a thousand hostages inside. It's not negligence, but murder," said Ella Kesayeva, who leads the Voice of Beslan victim support group.
Kulayev, who says he took part in the hostage-taking but is innocent of the charges, showed little reaction to the proceedings.
Authorities say Kulayev is the sole survivor of the 32 hostage-takers who seized the school.
The prosecutor has called for the death penalty, but Russia declared a moratorium on executions in 1996, meaning the harshest punishment would be life in prison.