The letter was sent to the Nya-Wermlands Tidningen newspaper, written in caps in poor English. It was signed by the "Anders Behring Breivik Sympati Group North Europa". Excepts below are copied verbatim.
"We will detonation a large bomb outside the residense of Karlstad. We inn our group have big sympaty four our leader Anders Beiring Breivik from Norway who gave ous big hope for the feuture," the letter read.
"We coming with 3 big bombs 1.5 tonne fertiliser… we give this informationn because the swedish polise is not so effectiv."
The group also claimed responsibility for the murder of Sweden's then-Prime Minister Olof Palme in 1986.
"A dead kind and queen is a good king and queen," the letter continued. "Also, homesexual and lesbian people will be our target inn the feature."
Swedish police are taking the threat seriously, however, and have cordoned off a large area around the Karlstad residence, where Sweden's King Carl XVI Gustaf and Queen Silvia are set to arrive on late Thursday afternoon.
"We have taken in bomb dogs, forensic teams are on the way, and we have investigators looking into it. We are treating this as a real bomb threat against the king's visit," Rolf Sandberg of the Värmland police told the Aftonbladet newspaper.
The police added that they see no threat to the public, and the royal visit has been scheduled to continue as planned.