Political desire to see more police on the street must not reduce the Metropolitan Police's overall capability, a report has cautioned.
The Metropolitan Police Authority wants to increase the percentage of officers on so-called "front-line" duty.
But the London Assembly report warned political commitments to protect front-line policing mean little because there is no popular definition of the term.
The Met said it would provide a balance of visible and specialist staff.
A spokesman said: "The Met has been very proactive in seeking to identify savings from areas such as its estate, vehicles and contracts so that it can prioritise operational policing services.
"In order to keep London safe it is necessary to maintain a balance of highly visible uniform officers and specialist investigators who have the additional skills to target rapists, child abusers, and drug and people traffickers.
"Finding the £600m savings that we are likely to have to make by 2015 is extremely challenging but the Met is committed to keeping the capital safe whilst it does so."
Currently officers fill 45% of operational support roles - some of which could be done by civilians.
The report found that money could be saved by using more civilians for these roles.
John Biggs AM, one of the report's authors, said: "Londoners need to know they are getting value for money from their police force but talk of funding cuts to such a vital service will understandably worry people.
"Any changes will need to be based on a better understanding by all parties - including the public - of how policing works now and what needs to happen in the future.
"It is not just about having more visible police on our streets, but ensuring that the Met is working in a way that makes it more effective and efficient at every level of its operations."
A spokesperson for London Mayor Boris Johnson said: "London is set to have a new pioneering model for policing with the establishment of The Mayor's Office for Policing and Crime, due later this year, pending the passage of the Police Reform and Social Responsibility Bill.
"In light of this development, the mayor has welcomed the London Assembly's report and will respond in due course."