Gardaí and the banks have warned people to be extra vigilant following a number of reported “shoulder surfing” and “dipping” incidents in Dublin.

As an anticipated 400,000 people are expected in the capital for Monday’s parade, a number of incidents of fraudulent use of ATM machines has already been reported to the Garda.

In what was described as “a typical” incident on Friday night a group of young women were watched by “shoulder surfers” as they paid for their food in a Grafton street restaurant. Shoulder surfers are people who look over customers’ shoulders when pin numbers are being used, or observe from a nearby point, possibly with a hidden camera.

After the women left the restaurant they were followed and a “dipper” was able to unzip a bag and remove a purse. Within an hour almost €1,000 had been taken in cash from a Grafton Street ATM.

An attack could leave a victim out of pocket to the tune of a daily limit on a laser-type card, twice that if it was used either side of midnight, or even up to the full credit limit on a credit card.

A Garda source said it was no secret that gangs of criminals would be in the city, mingling with tourists in a bit to spot card holders’details before stealing the card itself. “It happens everywhere, in every city at every festival, and people should always behave as if they are being watched when they use a cashpoint or even when paying for goods and services,” he said.

The garda said the shoulder surfing and dipping was the latest in a series of scams involving ATMs and customer details. Just last week gardaí broke up a gang believed to have scammed some €200,000 through 40 cash dispensers in the past few months.

Under an initiative called Operation Glint, headed up by a detective superintendent, gardai from the Dublin South Central division, the National Bureau of Fraud Investigation and officers from Naas, Co Kildare placed a number of suspects under surveillance. Armed officers stopped a car at Sallins Road, Naas on Wednesday and arrested two men from Moldova.

The Garda source said a number of people in suburban car parks had also been approached by a young woman asking if they had dropped a €10 note. “It usually ends with the person, usually elderly, discovering a day later that their card and money are gone,” he said.

A number of banks have been known to have reissued hundreds, if not thousands of cards in recent weeks as skimming machines were found on ATM machines. In these cases even if criminals were not successful in getting card pin numbers - because of the growing prevalence of people shielding their pin numbers - the card would be reissued.

Authorities said the perpetrators were highly mobile and people should be extra vigilant when attending festivals and events in town.