A shooting incident at an education centre in Sweden's Orebro city on Tuesday left at least 11 people, including the suspected gunman, dead, with Prime Minister Ulf Kristersson describing it as 'worst mass shooting' witnessed by the country in its history.

The attack happened at around 12.30pm local time (11.30am UK time) at Campus Risbergska in the town which is located around 200km (125 miles) west of Stockholm city.

The police believe that the key suspect acted alone.

The motive of the attack is still not known.

Police reportedly carried out investigations at several locations in the town following the attack.

Police has so far ruled out that the incident was connected with terrorism.

Police said officers are working out to identify the attacker and the victims.

The damage at the crime scene was so extensive that investigators were unable to be more definitive on the number killed, Roberto Eid Forest, head of the local police, told Sky News.

"When it comes to saying anything more about the perpetrator, it is still very early. The operation is ongoing and that will undoubtedly become clearer. But we are working very intensively right now," he said.

The suspected gunman had not previously been known to police, Forest said.

Swedish PM describes the attack as 'brutal violence'

Swedish PM Ulf Kristersson was quoted as saying by Sky News: "Today, we have witnessed brutal, deadly violence against completely innocent people."

"This is the worst mass shooting in Swedish history. Many questions remain unanswered, and I cannot provide those answers either," he said.

He said the motive behind the attack will be known with time and asked people not to speculate about the incident.

King Carl XVI Gustaf described the incident as a 'terrible atrocity'.

"My family and I would like to express our great appreciation for the police, rescue and medical personnel who worked intensively to save and protect human lives on this dark day," the King was quoted as saying by Sky News.

Six people were reportedly rushed to local hospital.

The number of injured people in the attack is still not clear.

According to reports, four people underwent surgery.

The shooting reportedly occurred at a time when many students had gone home after the national examination.

However, those who were present in the campus were evacuated.

Andreas Sundling, 28, was among those who were forced to barricade themselves inside the school.

Andreas told Sky News: "We heard loud bangs and first we thought people were fighting outside and maybe throwing chairs and tables, [but] then we heard people screaming."

"Then the people in my class realised that something is wrong. We closed all the doors to the classroom and we locked the doors and I barricaded the doors with tables and chairs,"  Andreas said.

Shooting and attack incidents on education institutes are rare in Sweden.