Google's plans to track users' locations on its free wireless internet access service in San Francisco have caused unrest among privacy campaigners. 

The system is seen as a backdoor way for government agencies or law enforcement bodies to follow people's movements within the city.

"The greatest concern is that once you have that treasure trove of information, will people start to come looking for it?" Kurt Opsahl, a staff attorney with the Electronic Frontier Foundation, told Associated Press. 

The city-wide Wi-Fi service is expected to be rolled out in two formats: a paid-for ad-free service costing $20 a month; and a free service that displays adverts.

In order to use the service, laptop owners will need to log-on with a Google account so that the company will know who they are and where they are.

Google stressed that it will use the tracking information to provide relevant advertisements to the users based on their location.