The incident occurred on Friday afternoon at the Brinell school, which has about 450 students aged 16 to 20, in Fagersta, a town of 13,000 inhabitants in central Sweden.
The school had just opened after the northern hemisphere summer recess and for some students it marked the first day at the senior high school.
"We received an alarm at 2.06pm about suspected deadly violence and sent all available resources to the school," a police spokesperson told Reuters.
The spokesperson declined to comment on whether anyone had been killed but said that one person at the scene was later taken into custody and that there was no longer any danger to the public.
Jonas Cederberg, director of health and medical services for the Vastmanland county authority, told Reuters two people had arrived for treatment and were "seriously injured".
"A third person is on the way. As they haven't yet arrived, the extent of their injuries remains unclear at this stage," Cederberg said.
Public broadcaster SVT reported based on unnamed sources that the person arrested was an 18-year-old former student at the school who had a previous conviction for assault.
He carried a sword and wore a helmet during the attack, SVT reported.
A student at the school, Abdulrahem Salmo, said he had seen a victim with a bloody stab wound in the arm.
"Then people called the police and we just all ran from there because we were afraid," he told SVT.
Local newspaper Fagersta-posten talked to Madelene Jansson outside the school, which her daughter had been attending.
"I first got a text message from her that somebody had been stabbed and that police was on its way. Then it all just went off. It feels unreal," she told the paper.
Several police officers were seen coming in and out of a police cordon surrounding the school.
Several other schools and public buildings in Fagersta, a town of 13,000 inhabitants, were at first placed in lockdown after the sword attack took place.
The lockdown was later lifted and a group of teenagers could be seen leaving Brinell school.
"We are moving into high alert status to marshal our resources and gain the clearest possible picture of the situation," the Vastmanland county authority said in a statement.
Citing unnamed sources, the daily Expressen said police had shot the suspect in the case but Reuters could not immediately confirm the report.
The incident comes a year after a school shooting in Orebro, also in central Sweden, when a gunman killed 10 people before turning the weapon on himself, in the worst mass shooting in Swedish history.
The Swedish government is in close contact with the police following Friday's attack, Justice Minister Gunnar Strommer said in a statement to Reuters.
Swedish Prime Minister Ulf Kristersson said police were working intensively on the case and asked everyone to let them and the emergency services work undisturbed.
"Our thoughts are with all those affected," he wrote in a post on social media X.
Social Democrat leader Magdalena Andersson, whose coalition holds a clear lead in opinion polls ahead of next month's parliamentary election, said that "all of Sweden stands together with Fagersta at this difficult time".
with AP
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