Police have arrested an 18-year-old man suspected of killing two people and wounding seven others in a Saturday shooting targeting restaurants packed with teenagers in southern Finland.

The gunman opened fire around 1:53am (local time) from a rooftop in the centre of Hyvinkaa, a city about 50 kilometres north of the capital Helsinki, aiming at two restaurants where a large number of young people had gathered, police said.

An 18-year-old woman was killed on the spot while eight others were injured, including two seriously. One of the victims, also 18, later died in hospital from his injuries.

A police officer aged 23 was also wounded as she left her vehicle after arriving at the scene, a police spokesman said.

Eyewitness Jani Kuronen told public television YLE that "shooting erupted suddenly".

"People started running. One person had a horrible wound on the face. I called an ambulance," said the man in his 20s.

Another witness, Santeri Silvenoinen, said he heard the gunshots and "then looking around me, I saw a woman lying on the ground and I started running".

Police did not identify the suspect, but said he was dressed in camouflage fatigues. He had no previous criminal record and did not resist his arrest close to Hyvinkaa, a city of 45,000 inhabitants.

Three firearms were seized from the suspect, who was not in possession of any gun permits, police said.

Asked about the gunman's motives, police spokesman Satu Koivo said an investigation was ongoing and that "for the moment, it was not possible to confirm anything".

However, the spokesman told AFP that police did not believe the gunman knew his victims.

Hyvinkaa mayor Raimo Lahti told YLE that the whole town was in shock.

In recent years Finland has been hit by several cases of deadly shootings by teenagers.

On September 23, 2008, a student who went on a murderous rampage at a school at Kauhajoki in the south-west killed 10 people before turning his gun on himself.

In a similar incident on November 7, 2007 at Jokela, not far from Hyvinkaa, an 18-year-old killed seven classmates and the headmistress then shot himself in the head.

AFP