A 12-year-old opened fire Tuesday at a school outside the Finnish capital Helsinki injuring three other children, police said, adding that the attacker was in custody.

The school in Vantaa has around 800 pupils and 90 staff. Children in grades one to nine, or aged seven to 15, attend the school.

"There are people injured in the shooting incident", police said shortly after 10:00 am (0700 GMT).

Police were alerted to the scene at 09:08 am and later specified that both the suspect and the injured were all 12 years old.

The city of Vantaa's crisis group was activated following the shooting, local media reported.

Images from the scene showed a large number of police at the school.

In an update around 11:30, police said the suspect, who was carrying a gun at the time, had been arrested in an area of Helsinki in a "calm manner."

Parents of pupils attending the school told media that the shooting had occured in a classroom.

Police urged the public to stay away from the area and remain indoors.

"Do not open the door to strangers," they said in a statement.

Finnish Interior Minister Mari Rantanen said in post to X that the day had started in a "shocking way".

"I can only imagine the pain and worry that many families are experiencing at the moment," she said.