The blast in a building at the Tingvalla high school was felt across the town of Karlstad in the south west of the country just after 3pm local time (2pm GMT).

A fire was then reported at the site.

No casualties have been reported so far.

Student, Eleven Daniel, was in a lesson in the building when the blast went off.

He said: "There was a massive explosion. The windows and parts of the walls began shaking."

Witnesses reported seeing a flash of light at the time of the blast which was promptly followed by smoke coming from the roof of the building at the high school.

Headteacher Christine Holmberg said: "Despite being metres away from the building I could feel it shaking.

"Suddenly a janitor came running out saying there was a bomb in the gym. By the time we arrived at the gym emergency services were already on site."

There is currently no visible evidence to indicate that an explosion has taken place.

She continued: "There is no evidence.

A teacher, who was in the staff room inside the building at the time, told a local paper: "There was a very loud bang. We thought it was a bomb."

And a message on the school's Facebook page said: "No one has been injured in the explosion outside Tingvallagymnasiet which occurred today shortly before 15.00.

"Teaching continues as usual during the afternoon. Those students who want and need to get support from the staff of the school can.

"Emergency services are investigating what could have caused the blast.

"They have found no explanation. Their theory is that someone caused the blast then went off but has not left any trace behind."

Ola Akesson, head of operations at Karlstad’s emergency services, said: “We are on site now and are trying to ascertain what has exploded.

"It was a powerful explosion."

She added the "bang" could be heard in other parts of the city.

Ms Åkesson later added: "We left the scene a while back.

"In our opinion some sort of explosion occurred outdoors. No one was injured and there was no damage to property or real estate.

Police confirmed they are looking for two men after the "explosion".

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The blast could be felt across the town of Karlstad

The incident comes just days after police rushed to central Stockholm after reports of a "powerful explosion" in the capital.

According to witnesses a massive blast could be heard in a mall near the Government building and a mysterious car was spotted on site.

An eyewitness said: "We were inside when we heard the bang. The entire mall shook.

"We saw people running out, so we rush out too. Once outside we saw a big white cloud of smoke."