MONTGOMERY, Ala. - The number of extremist groups in the United States exploded in 2009 as militias and other groups steeped in wild, antigovernment conspiracy theories exploited populist anger across the country and infiltrated the mainstream, according to a report issued today by the Southern Poverty Law Center (SPLC).

Antigovernment "Patriot" groups - militias and other extremist organizations that see the federal government as their enemy - came roaring back to life over the past year after more than a decade out of the limelight.

The SPLC documented a 244 percent increase in the number of active Patriot groups in 2009. Their numbers grew from 149 groups in 2008 to 512 groups in 2009, an astonishing addition of 363 new groups in a single year. Militias - the paramilitary arm of the Patriot movement - were a major part of the increase, growing from 42 militias in 2008 to 127 in 2009.

The report, "Rage on the Right," is the cover story in the Spring 2010 issue of the SPLC's quarterly investigative journal Intelligence Report.

Patriot groups have been fueled by anger over the changing demographics of the country, the soaring public debt, the troubled economy and an array of initiatives by President Obama that have been branded "socialist" or even "fascist" by his political opponents.

"This extraordinary growth is a cause for grave concern," said Intelligence Report editor Mark Potok. "The people associated with the Patriot movement during its 1990s heyday produced an enormous amount of violence, most dramatically the Oklahoma City bombing that left 168 people dead."

The Patriot movement has made significant inroads into the conservative political scene, according to the new report. "The ‘tea parties' and similar groups that have sprung up in recent months cannot fairly be considered extremist groups, but they are shot through with rich veins of radical ideas, conspiracy theories and racism," the report says.

Unlike the 1990s, the Patriot movement's central ideas are being promoted by people with large audiences, such as FOX News' Glenn Beck and U.S. Rep. Michele Bachmann of Minnesota. Beck, for instance, reinvigorated a key Patriot conspiracy theory - the charge that the Federal Emergency Management Agency is secretly running concentration camps - before finally "debunking" it.

The growth of Patriot groups comes at a time when the number of racist hate groups stayed at record levels - rising from 926 in 2008 to 932 in 2009, according to the report. The increase caps a decade in which the number of hate groups surged by 55 percent. The expansion would have been much greater in 2009 if not for the demise of the American National Socialist Workers Party, a key neo-Nazi network whose founder was arrested in October 2008.

There also has been a surge in "nativist extremist" groups - vigilante organizations that go beyond advocating strict immigration policy and actually confront or harass suspected immigrants. These groups grew from 173 groups in 2008 to 309 in 2009, a rise of nearly 80 percent.

These three strands of the radical right - the hate groups, the nativist extremist groups, and the Patriot organizations - are the most volatile elements on the American political landscape. Taken together, their numbers increased by more than 40 percent, rising from 1,248 groups in 2008 to 1,753 last year.

There are already signs of radical right violence reminiscent of the 1990s. Right-wing extremists have murdered six law enforcement officers since Obama's inauguration. Racist skinheads and others have been arrested in alleged plots to assassinate the president. Most recently, as recounted in the new issue of the Intelligence Report, a number of individuals with antigovernment, survivalist or racist views have been arrested in a series of bomb cases.

The hate groups listed in this report include neo-Nazis, white nationalists, neo-Confederates, racist skinheads, Klansmen and black separatists. Other hate groups target gays or immigrants, and some specialize in producing racist music or propaganda denying the Holocaust. A list and interactive, state-by-state map of active hate groups can be viewed here.