The American legal thriller writer, 59, said a lot of "guys my age" were being locked up when they had done no more than "had too much to drink" and visited an illegal website.

He attacked the American judicial system in an interview with London's Telegraph, saying that judges had "gone crazy" over the past 30 years, locking up far too many people, from white-collar criminals, such as the businesswoman Martha Stewart, to black teenagers on minor drugs charges.

"We have prisons now filled with guys my age. Sixty-year-old white men in prison who have never harmed anybody, would never touch a child," Grisham said.

"But they got online one night and started surfing around, probably had too much to drink or whatever, and pushed the wrong buttons, went too far and got into child porn." The author of thrillers such as The Firm and A Time to Kill, who has sold more than 275 million books during his 25-year career, cited the case of a "good buddy from law school" who was caught in a Canadian child pornography sting operation a decade ago as an example of excessive sentencing.

"His drinking was out of control, and he went to a website. It was labelled 'sixteen-year-old wannabe hookers' or something like that. And it said '16-year-old girls'. So he went there. Downloaded some stuff - it was 16-year-old girls who looked 30.

"He shouldn't have done it. It was stupid, but it wasn't 10-year-old boys. He didn't touch anything. And God, a week later there was a knock on the door - 'FBI!' - and it was a sting set up by the Royal Canadian Mounted Police to catch people - sex offenders - and he went to prison for three years.

"There's so many of them now. There's so many 'sex offenders' - that's what they're called - that they put them in the same prison. Like they are a bunch of perverts, or something; thousands of them. We've gone nuts with this incarceration," he added.

Asked about the argument that viewing child pornography fuelled the industry of abuse behind the creation of the pictures, Grisham said that current sentencing policies failed to draw a distinction between real-world abusers and those who downloaded content, accidentally or otherwise.

"I have no sympathy for real paedophiles," he said, "God, please lock those people up. But so many of these guys do not deserve harsh prison sentences, and that's what they're getting."

He added that sentencing disparities between black people and white people was likely to be the subject of his next book.

There are currently 2.2 million people in American prisons, more than 750 per 100,000 population. This makes the US by far the heaviest user of prison sentences in the world. By contrast, Britain imprisons just 154 per 100,000 population.

Grisham's remarks are likely to anger child-rights campaigners who over the past decade have successfully lobbied the US Congress to demand tougher sentences for those who view child pornography online.

Since 2004, average sentences for those who possess but do not produce child pornography have nearly doubled in the US, from 54 months in 2004 to 95 months in 2010, according to a report two years ago by the US Sentencing Commission.

The issue of sex-offender sentencing has created some debate among the American legal community after it emerged that in some cases those who viewed child pornography online were at risk of receiving harsher sentences than those who committed physical acts against children.

A provocative article in the libertarian magazine Reason headlined "Looking v Touching" argued last February that something was "seriously wrong with a justice system in which people who look at images of child rape can be punished more severely than people who rape children".

In January this year, the US Supreme Court was unable to resolve a debate over whether a man who viewed images of a child rape should be as liable to pay the same financial compensation to the victim as the perpetrator of the crime. Grisham, who earned $17 million ($19.3 million) from his work last year, is one of America's highest-paid novelists and is a self-declared Democrat who supported Hillary Clinton in her failed 2008 attempt to win the White House.

He has waded into political issues in the past, writing newspaper columns that argued against Guantanamo Bay prison camp and the death penalty as well as serving on the board of the Innocence Project, a campaign group that uses DNA analysis to end miscarriages of justice.

Grisham gave the interview to promote Gray Mountain, his latest novel in which a young lawyer takes on "Big Coal" as it destroys the rural landscape of Virginia.

He also contended that Barack Obama had presided over "amateur hour" at the White House during his six years as president, and that Amazon.com was seeking to establish an effective online monopoly that could ultimately destroy the paperback books business.

Telegraph, London