Bill Clinton today warned politicians and commentators to tone down their rhetoric for fear of inflaming hate groups and provoking violence, as a poll suggested that public trust in the US government is at its lowest point for half a century.

Amid growing concern in the White House about the anti-government mood and a marked rise in radical fringe groups, Clinton said the internet made it easier to spread ideas to reach "the unhinged". The worry is not so much over populist movements such as the Tea Party but the revival of extreme groups that have been encouraged by general anti-government sentiment.

On the 15th anniversary of the 1995 Oklahoma bombing, the worst terrorist attack in the US before 9/11, Clinton wrote in the New York Times: "We are again dealing with difficulties in a contentious, partisan time … As we exercise the right to advocate our views, and as we animate our supporters, we must all assume responsibility for our words and actions before they enter a vast echo chamber and reach those both serious and delirious, connected and unhinged."

Clinton said it should not be forgotten what drove the Oklahoma bombers. "They took to the ultimate extreme an idea advocated in the months and years before the bombing by an increasingly vocal minority: the belief that the greatest threat to American freedom is our government and that public servants do not protect our freedoms but abuse them."

At a ceremony in Oklahoma yesterday, survivors and victims' relatives commemorated the 168 people who were killed.

The extent of the anti-government mood in the country was revealed today in a Pew Research Centre poll that found public confidence in the federal government was at its lowest for half a century. Almost eight out of 10 Americans surveyed said they did not trust it; only 22% said they trusted the federal government almost always or most of the time.

Andrew Kohut, director of the Pew Centre, said: "Trust in government rarely gets this low. Some of it is backlash against Obama. But there are a lot of other things going on."

The poll identified several reasons for the rise in anti-government feelings: opposition to Obama; the recession, in particular the role of Wall Street; and anger with both Democratic and Republican members of Congress.

The Southern Poverty Legal Centre, which tracks hate groups, said yesterday there has been a surge in the number of militia and other extremists. Mark Potok, its head, said the mood was so volatile that it needed only a spark to set it off.

Last month nine people linked to the Hutaree, a Christian militia group in Michigan, were arrested over an alleged plot to kill police in the hope of starting an uprising against the government. In February, a lone anti-tax protester flew a plane into the tax office in Austin, Texas.

Members of Congress who last month supported the health reform bill have been targets of vandalism and death threats. Yesterday several hundred pro-gun activists gathered at the Washington Monument to demand the end of restrictions on carrying guns in public.

Larry Pratt, executive director of Gun Owners of America, said groups such as his were set on ousting moderate Republicans in primaries before the November Congressional mid-term elections.

"We are in a war," Pratt, a former Republican member of the Virginia state legislature, told a rally. He said the Obama administration was out to take their freedom, their money and their childrens because it was socialist.

A newly-formed group, the Oath Keepers, made up of former members of the armed services and police forces, had been due to take part in a rally in Virginia at which they would openly carry firearms but pulled out, expressing fears of confrontation with the police.