Police in South Shields did not release the name of the man arrested Saturday, The Journal of Newcastle reported. But he was identified online Monday as Kenny Holden, 29, of South Shields.
The threat was made on Facebook pages linked to the EDL, investigators said. Holden allegedly said he would carry out the "Oslo-style" attack in South Shields.
Anders Behring Breivik, a Norwegian nationalist, is on trial for killing eight people in a bombing at a government building in Oslo and 69 more in a shooting spree at a summer camp run by a left-of-center party on Utoya Island near the capital.
Holden has been released on bail.
"Extensive inquiries are being carried out. There is no place for any sort of racial incitement or use of social networking to place inappropriate comments and we take incidents such as this extremely seriously," Chief Inspector Michael Barton of the Northumbria Police said.