The number of anti-government, far-right extremist groups has soared to record levels since 2008 and they are becoming increasingly militant, according to a report by the Southern Poverty Law Center.

It says the number of groups in the "Patriot" movement stood at 1,360 in 2012, up from 149 in 2008 when Barack Obama was first elected president, an increase of 813%. The report said the rise was driven by opposition to Obama and the "spluttering rage" over federal attempts at gun control.

Those who were identified as "militia" groups or the paramilitary wing of the Patriot movement, numbered 321, up from 42 in 2008, the SPLC said in its report.

Concern over a "truly explosive growth" of groups on the radical right, along with a rise in domestic terrorist plots, has prompted the SPLC to write to US attorney general Eric Holder and Homeland Security secretary Janet Napolitano, warning of the potential for domestic terrorism and urging them create a new, inter-agency task force to assess whether it has adequate resources to deal with it.

The report says that the numbers far exceed the "high-water mark" of 820 groups in 1990s when the rise in militias was fuelled by the Waco siege, the Brady Bill and the 1994 assault weapons ban.

Richard Cohen, the SPLC president and a member of the Department of Homeland Security's group to counter violent extremism, wrote in the letter: "On October 25, 1994, six months before the Oklahoma City bombing, we wrote attorney general Janet Reno about the growing threat of domestic extremism. Today we write to express similar concerns.

"As in the period before the Oklahoma City bombing, we now are seeing ominous threats from those who believe that the government is poised to take their guns."

Timothy McVeigh drove a truck full of explosives into a federal building in Oklahoma City in April 1995, killing 168 people, 19 of them children under six, and injured hundreds more.

"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."

Potok said that the demographic factors driving the rise in such groups began before Obama became president – the census bureau predicts that whites will become a minority group in the US by 2043 – but have been fuelled by the changes in America he represents. The growth in extremism has been helped by the "successful exploitation over illegal immigration" and by anger over the gun control debate, he said.

Law enforcement officials have uncovered numerous terrorism conspiracies born in the militia subculture, including plots to spread poisonous ricin powder, to attack federal installations, and to murder federal judges and other government officials, the report says.

Potok cited a study by the Combating Terrorism Center at the West Point military academy, which found that right-wing violence in 2000-2011 surpassed that of the 1990s by a factor of four. He expected extremism to rise, as anger over gun control had become a "grassroots rebellion". He said that 20 states are considering laws that would aim to nullify federal gun control measures and 500 sheriffs mainly in western US, who say they will not enforce any such measures.

Daryl Johnson, a former Department of Homeland Security official, said in a press call that SPLC's numbers were likely to be a "on the conservative end" because they did not include clandestine and underground groups which did not have a presence on the internet.

Johnson, who was a member of the now-disbanded non-Islamic terrorism unit at the Department of Homeland Security, authored a report in 2009 warning about the increasing dangers of right-wing extremism which created a political firestorm, and was later withdrawn. He said it was "quite unsettling" that nothing had changed at the DHS in the last four years despite the rise in extremism.

Although only a small pool of individuals associated with such groups were potentially violent, and radicalisation was difficult to analyse, Johnson said: "This pool of potentially violent extremists should raise a red flag of concern."

He urged FBI and local law enforcement officials to assess the threat, and said more analysis was needed.

The SPLC's report on hate and extremism, contained in its quarterly intelligence report, also found that hate groups remained at a near-record level of 1,007 groups in 2012, a slight drop from the 1,018 groups documented in 2011.

SLPC defined "Patriot" groups as those who believe that the federal government is engaged in a conspiracy, is prepared to engage in martial law, would take away guns and would force the US into some kind of so-called "One World Nation".

The hate groups listed in this report include neo-Nazis, white nationalists, neo-Confederates, racist skinheads, Klansmen and black separatists. Other hate groups on the list target gay people, Muslims or immigrants, and some specialise in producing racist music or propaganda denying the Holocaust. © Guardian News and Media 2013