Säpo said the four people had been arrested during an operation conducted in cooperation with the police in the Stockholm area.

“The persons are suspected of preparations for terrorist offences and aggravated weapon offences,” Säpo said in a statement.

A number of locations were searched during the operation.

“The case concerns violent Islamist extremism,” Säpo continued, adding that there were also links to organised crime.

In a separate statement, the Swedish Prosecution Authority confirmed the arrests and said an investigation was being headed by a prosecutor from its security unit.

It said that it could not provide further details on the case at this moment.

Newspaper Expressen said that local residents had reported hearing several loud bangs as police stormed buildings in a southern suburb of Stockholm.

Expressen said the police’s National Task Force had likely used shock grenades as it entered the premises of an Islamic organisation in the suburb of Tyresö.