The tests weed out "bots" by ensuring that users are human. But Google has been accused of exploiting the data submitted in by the public for commercial gain - by adding the information to its own mapping system.
Campaign groups said that the use of pictures of real house numbers presents “serious” security issues, and accused the internet company of being “underhand and crude”.
The pictures of house numbers, which are taken from doors and fences on its Street View mapping service, appear on Google’s websites when internet users are asked security questions in order to access their accounts.
In order to gain access to the page, web users are asked to identify a blurry house number by typing it into a box.
The same image is presented to other Google users around the world at the same time. If enough people submit the same number, Google accepts they have accurately read the photo and are therefore not bots.
Traditionally these security checks – of which there are estimated 200 million a day - have involved typing blurred letters or words into a box.
Nick Pickles, director of privacy and civil liberties at Big Brother Watch, condemned the use of pictures of real house numbers as security questions.
“There is a serious privacy issue with identifying the individual number of people’s homes,” he said.
Mr Pickles also accused Google of using the pictures to further its own interests.
This is because security questions that involve typing a word into a box actually form part of the technology company’s Google Books project – which aims to digitize thousands of physical books that were written before the computer age.
When people type in garbled words they are helping the technology company to translate scanned images from old books where the ink used has bled or faded, meaning that the words can not be recognised by a computer programme.
Mr Pickles said that there is no such ‘public interest’ use in retyping house numbers.
Rather, Google uses the affirmative identification of a house number to sharpen up the image on its Street View or Google Maps service.
"It is clear that Google sees the people who use its services as a commodity to be used up. To use the public as unwitting data loggers is both underhand and crude.
"The 'Don't be evil' mantra appears to have been replaced with a thirst for knowledge," he said.
A Google spokesman confirmed that it has launched a global trial using house numbers as security questions. He said that the pictures are only used in 10 per cent of all questions.
The spokesman said that there are no security risks as the pictures of the numbers are cropped very closely. All pictures are taken from public roads and highways, he added.
When someone types the number in correctly, Google will then sharpen up the online image, the spokesman said.
“We are currently running an experiment in which characters from Street View images are appearing in CAPTCHAs [the security system that Google owns].
“We often extract data such as street names and traffic signs from Street View imagery to improve Google Maps with useful information like business addresses and locations,” the spokesman said.
Google said that it runs hundreds of trials such as this all the time. The aim is to make gaining access to websites safer, the spokesman said.
The point of security systems such as CAPTCHA is to stop legitimate websites being flooded with unwanted internet spam.
