Police said earlier that all three victims were 12 and that a suspect, also aged 12, had been detained.

Parents told Finnish media that the shooting had taken place in a classroom at Viertola school in Vantaa to the north of the capital Helsinki.

Police said they arrived at the school within nine minutes at 09:17 (06:17 GMT) and tended to the three victims.

One of the children died at the scene, they added.

The suspect ran off as soon as police arrived and was eventually detained "in a calm manner" in the northern Siltamaki district of the capital Helsinki at 10:00.

Police said he had been holding a firearm which they had taken from him and he had admitted carrying out the shooting in an initial interview.

They have now opened an investigation into murder and attempted murder.

In common with other Finnish schools, children had just returned to classes in Vantaa, just outside Helsinki, after the long Easter weekend.

Children were told to stay in their classrooms after the attack, public broadcaster YLE reported.

Prime Minister Petteri Orpo described the shooting as deeply upsetting and said his thoughts were with the victims and their families as well as everyone at the school.

Vantaa is Finland's fourth biggest city with some 240,000 residents.

Viertola school has 800 students aged seven to 16 of both primary and middle-school age on two separate sites, with some 90 staff. The shooting took place at at the school's Jokiranta site where pupils aged 9-13 are taught.

Initially police said everyone involved was 13 but then revised their ages down to 12.

As news emerged of the shooting, parents gathered at the school to pick up their children, although the building where the incident took place remained cordoned off.

Finland saw two deadly school shootings in a matter of months in 2007 and 2008.

In 2007, an 18-year-old student shot dead six pupils, the school nurse and his head teacher in the small town of Jokela north of Helsinki, then the following year another student shot dead nine pupils and a teacher with a semi-automatic rifle at a polytechnic in the western town of Kauhajoki.

The shootings prompted a tightening of gun laws, requiring gun owners to be at least 18, but anyone over 15 can apply for a permit to use someone else's weapon.

Finland is widely known as a country of hunters and gun enthusiasts and has 430,000 licensed gun owners in a population of 5.5 million, according to government statistics. There is no limit to the number of guns that can be owned and the interior ministry says more than 1.5 million are in circulation.