The Met Police has made record requests for data on London commuters, a majority of whom use Oyster smartcards, the Green Party has said.
Transport for London (TfL) figures show the Met made 6,576 requests in 2010, but it was turned down 810 times.
Noel Lynch, chairman of London Green Party, called for "rigorous safeguards to protect people's privacy".
The Met said the rise in requests was due to the rise in Oyster usage as the data helps trace a person's movement.
Figures obtained by the Green Party from TfL show that in 2007 the Met made 4,939 requests, but 747 of those requests were turned down.
The number of requests rose to 6,074 in 2008, of which 1,279 were turned down. In 2009 police requests fell to 5,619 in 2009, but the Met were denied information on 918 instances.
The requests soared to 6,576 in 2010 until the end of October, but TfL turned down fewer police requests (810) - the lowest since 2008.
Mr Lynch said: "The vast majority of these requests are for Oyster card data.
"While this information may have a role to play as an investigative tool in certain circumstances it is vital that there are rigorous safeguards to protect people's privacy."
"There is clearly a risk that TfL could be overwhelmed with requests or otherwise pressured into handing over personal data without sufficient checks."
Request rise 'proportionate'
A Met spokesman said: "Each case is looked at individually but such information could be used to build up a picture of a person's movements.
"If a line of inquiry was identified that required Oyster card data then the officer would request the information from Transport for London.
"As Oyster cards become more widely used, it is likely that such requests will rise in proportion with their usage."
A TfL spokeswoman said: "Law enforcement, including police, requests for information on the Oyster system are subject to strict rules and procedures. Each request must relate to a specific police investigation.
"TfL considers each and every request on a case-by-case basis and in accordance with the Data Protection Act.
"Only a few authorised individuals within Transport for London can access this data."