France violates human rights in the way it handles terrorism-related cases, a Human Rights Watch report says.
The pressure group says France uses a catch-all offence to charge suspects even when they have only a vague link to an alleged terrorist organisation.
The report also says suspects can face long periods of detention before trial, and some have suffered physical violence during interrogation.
HRW says ministers need to take action or risk alienating some communities.
The BBC's Hugh Schofield, in Paris, says France prides itself on having perhaps the most effective anti-terrorist system in Europe.
The country has a team of specialist magistrates operating in close contact with the intelligence services, and an armoury of finely-honed laws to tackle the threat of terrorism, our correspondent says.
Since the mid 1990s there has been no serious terrorist attack.
But according to HRW, that level of security comes at the cost of some important breaches of natural justice.
The prime focus of the group's displeasure is the catch-all criminal charge under which the vast majority of terrorist suspects are held and tried.
The offence of "criminal association in relation to a terrorist undertaking" is excessively vague, HRW says.
It means that people face prosecution because of the flimsiest of links to an alleged terrorist operation.
The other main criticism concerns the way suspects are treated once in custody.
They cannot see a lawyer for three days, and then for only 30 minutes.
After being presented before a judge, they can be locked away in pre-trial detention for months or even years as the case against them is compiled.
HRW also says it has evidence of mistreatment of prisoners, including sleep deprivation, psychological pressure and physical abuse.
It urges the French government to take steps to address these problems, or risk alienating the communities from which future terrorist groups could emerge.