Thousands of American parents, worried that their children will be abused by paedophiles while trick-or-treating this Hallowe’en, have downloaded software to identify the homes of sex offenders in their neighbourhoods.

The Offender Locator application uses a GPS system built into Apple iPhones to pinpoint the user’s location, and then provides a map of sex offenders in the area. Leslie Gharghetta, a mother of two from Woodridge, New Jersey, uses it to monitor her neighbourhood. “I try and keep an eye on my kids. With Hallowe’en, I am able to see if anybody on the list is located near me,” she said.

The software has been downloaded more than a million times since becoming available in June. Tapping any location marked brings up a photograph of the offender, as well as their address, date of birth and convictions. “There is one offender listed two streets away from me,” said Mrs Gharghetta, 31, whose husband is a police officer.

“This tool helps you understand what is going on in your neighbourhood, who’s coming or going. People should be alert and know what is going on.”

The controversial software, criticised by privacy groups as a tool for vigilantes, has been in the top ten lists of iPhone applications since its launch, and the company behind it is expecting another boost before the trick-or-treat season.

All 50 states in the US require those convicted of sexual offences to sign a register so their location can be monitored. There are up to 700,000 sex offenders on these lists. The Megan’s Law sex offender list scheme was introduced in 1994 after a seven-year-old girl, Megan Kanka, was kidnapped, raped and murdered by a repeat violent sexual offender. Moves to create a similar registry in the UK have been resisted by the police, probation services, children’s charities and social services.

American parents use the iPhone application to generate lists of offenders by their proximity to schools, friends’ houses or parks.

The case of Phillip Garrido, a California sex offender accused of kidnapping and holding Jaycee Lee Dugard prisoner in his backyard for 18 years, prompted fresh fears about paedophiles in the community. On the registry for Antioch in California, where he lived, there were 120 sex offenders within a five-mile radius.

Trip Wakefield, chief executive of ThinAir Wireless, which developed the software, said parents worry that Hallowe’en is “primetime for sexual predators”. The company said that up to 10,000 people a day use the application. “The more light we shed on this issue, the better, and hopefully we can reduce the frequency at which these crimes occur,” said Mr Wakefield.

Legal and privacy experts warn that the information supplied may be out of date. Some states’s registries are months behind. Other critics point out that the vast majority of sexual attacks on children are carried out by family members, not strangers.

One review on the iTunes site posted by somebody claiming to be a father of young children who had mistakenly had sex with a minor, and was on an offender list, said:“People are downloading this thinking they are safe. But ... over 90 per cent [of offenders are] family members or close family friend. Wake up people.”