Serbia is struggling to come to terms with the deadliest school shooting in its history which saw a 13-year-old suspect kill eight fellow students and a security guard Wednesday.

The entire nation was in mourning Thursday a day after the shooting took place at a school in the capital Belgrade.

The tragedy has left the Balkan nation reeling. Gun violence at schools is extremely rare in Serbia and the country's president called Wednesday's shooting "one of the most difficult days" in recent history.

On Thursday, schools across Serbia held a moment of silence, while a leading mental health institute in Belgrade opened a crisis hotline to provide psychological support to students, teachers and the families affected by the shooting.

"I cried all day yesterday. My son went to school here," Mileva Milosevic, an 85-year-old retired lawyer living near the school, told AFP.

"I'll never forget this, because I have to walk here every day, while I can."

The Vladislav Ribnikar elementary school in downtown Belgrade's Vracar district remained sealed off Thursday, with police guarding the entrance to the building.

People continued to place flowers, toys and light candles at a makeshift memorial on the pavement outside of the school.

"Where are we as human beings, where is our empathy? Where did we all fail to see the problem, both with a person who did this and with all the other people that have led this to happen?" asked Belgrade resident Ana Djuric, 37, as she walked near the school.

Health officials said two of the seven injured were in critical condition after undergoing a string of operations.

Serbia will observe three days of mourning starting Friday. It comes at what is usually a festive time with people flocking outdoors to celebrate spring

The interior ministry has asked firearm owners to keep their guns locked in safes – warning that people who did not abide would have their weapons seized.

During a national address hours after the shooting, President Aleksandar Vucic proposed a string of measures to curb gun violence, including a two-year moratorium on issuing permits for firearms.

The president also said the suspected shooter – a student at Vladislav Ribnikar – would be held in custody in a "psychiatric ward," after prosecutors said he would not stand trial as he was a minor.

Authorities said he was armed with two pistols – one in his backpack and one that he used.

Belgrade police chief Veselin Milic said the suspect "planned the shooting for a month and made a list of kids he planned to kill."

"The sketch looks like something from a video game or a horror movie, which indicates that he planned in detail, by classes, who to liquidate," he added.