The Southern Poverty Law Center said in an Intelligence Report the number of these groups reached an all-time high of 1,360 in 2012.
"We are seeing a real and rising threat of domestic terrorism as the number of far-right anti-government groups continues to grow at an astounding pace," said Mark Potok, SPLC senior fellow and author of the report. "It is critically important that the country take this threat seriously. The potential for deadly violence is real, and clearly rising."
The center said it has sent a letter to Attorney General Eric Holder and Homeland Security Secretary Janet Napolitano urging the creation of a new interagency task force devoted to the threat of domestic terrorism posed by these "patriot" groups.
"As in the period before the Oklahoma City bombing, we now seeing ominous threats from those who believe that the government is poised to take their guns," SPLC President Richard Cohen wrote.
SPLC's tracking of radical right "patriot" groups indicates there were 149 in 2008. The following year the number more than doubled to 512.
The report said the increasing number of conspiracy-minded groups was "sparked by the election of America's first black president and anger over the economy."