US police departments are rapidly expanding the use of automatic license plate readers, sparking debate on whether the technology is a valuable crime-fighting tool or a massive invasion of privacy.

A recent report by the American Civil Liberties Union (ACLU) warned that these readers -- used in patrol cars or fixed locations on streets and highways -- collect data on tens of millions of Americans who have committed no wrongdoing, with a potential for privacy abuses.

The devices scan license plate numbers and match these against databases to help police locate stolen cars, criminals or missing children. Backers say this can free police officers from a monotonous task and help solve crimes.

But with many Americans uneasy over government surveillance of the Internet, the expansion of this technology has sparked concerns about Big Brother.

"In our society, it's a core principle that the government doesn't watch people's innocent activities just in case they may connected with a crime," said Allie Bohm of the ACLU.

"In many cases, police are retaining this data indefinitely with few privacy protections. The tracking of people is an invasion of privacy. It can reveal people's political views, religious activities and a lot of other personal information."

The ACLU report, based on a survey of hundreds of US police departments, said almost three quarters of police agencies reported using license plate readers, and 85 percent planned to increase their use.

Only a tiny fraction of the license plate scans helped point to crimes or stolen vehicles, according to the ACLU survey.

It found that for every million plates read in the eastern state of Maryland, only 47, or 0.005 percent, were potentially associated with a stolen car or a person wanted for a serious crime.

There have already been abuses. In one reported case, a mayor asked police to track his challenger to expose a relationship with a mistress. In another, police scanned the plates of people at a political protest and then investigated them.

Few oppose using the technology to fight crime, but the ACLU and others say keeping data on millions of people for years, or indefinitely, can be troublesome.

The report said private companies may end up holding this data with no oversight or privacy protections, noting that one firm holds over 800 million license plate location records from 2,200 law enforcement agencies, including the US Department of Homeland Security.

"We don't object to the use of these systems to flag cars that are stolen or belong to fugitives, but these documents show a dire need for rules to make sure that this technology isn't used for unbridled government surveillance," ACLU staff attorney Catherine Crump, the report's lead author, said in a statement.

But David Roberts, who heads the technology center for the International Association of Chiefs of Police, said these devices have become "enormously valuable" in fighting and preventing crime.

"It automates what is a time-consuming process which officers do on a manual basis," Roberts told AFP.

"These can trigger automatic alerts. And this can have extraordinary value in locating vehicles wanted for a variety of reasons."

He noted that the technology can help in "noncriminal" cases such as locating elderly people who may be suffering from dementia.

Roberts said surveys by the association indicate around 75 percent of US police departments are using or plan to use license plate scanners.

And he said the technology is widely used in other countries, notably Australia, Britain and Canada.

Police departments are aware of privacy concerns, but Roberts said these can be minimized by having guidelines in place on use and access of the data, with "strict audits" to ensure that police don't user the data for "fishing expeditions."

The association does not recommend a specific length of time to retain data, but urges police department to have policies that allow access only for official law enforcement purposes.

"It's not accurate to say this is a tracking system." he said.

"What these produce is an image of a license plate in a public space. You still need to access motor vehicle records to find out who the registered owner is."

Authorities appear to be listening in some cases.

This year, Virginia's attorney general ruled that police may only use the technology for "active" criminal investigations. And Rockville, Maryland agreed to a system to share its data with a state agency that deletes the information after one year.

Even as the debate rages, it remains unclear how effective the technology has been in reducing or solving crime.

A 2010 study led by Cynthia Lum at George Mason University was unable to determine whether license plate readers helped prevent auto theft or other crimes in auto crime hot spots.

Lum, a former police officer, said the study was limited in scope. She is seeking to conduct a comprehensive study on the impact on overall crime from the technology.

Lum, who heads the university's Center for Evidence-Based Crime Policy, said the technology is appealing because "it automates a process of investigation that police have been using for many years."

But she noted that the evidence on the effectiveness of scanners "is still underdeveloped."

"There is a chance you might acquire this technology and it might not give you the value in crime prevention that you anticipate," she said.