The reforms were supposed to be implemented in June. But a French court ruled on Friday that they had to be applied right away.
The European Court of Human Rights condemned garde à vue last year, in an appeals case in which a defendant had not been informed of his right to silence.
The French parliament passed a reform of the procedure earlier this year, which will go into effect this weekend.
Now, someone taken into police custody has to be informed of their right not to say anything. And they have the right to a lawyer during all interrogations.
Previously, lawyers were allowed in the first 30 minutes only, and could not meet with their clients until 48 or 96 hours later, delaying their ability to build a proper defence.
Lawyers will be allowed to attend all police interviews, and even ask questions themselves.
Many defence attorneys welcome the reforms as a step towards individual liberties – though some say it does not go far enough. Some say the reforms will put a strain on public defenders. Some lawyers have already signed up to volunteer this weekend.
Police have complained it would add to their workload. There are likely to be protest events throughout the weekend.
Aurelien Hamelle, a criminal defense attorney in Paris, welcomes the reforms
"For the first time, the right to defend oneself and the right not to incriminate oneself will actually be effective in the police office, and for France this is a revolution. France will finally have joined the advanced groups of nations in respect of individual freedoms in the police office.
"It’s really a shame that the police have not supported this reform, because if you look at how criminal trials take place today in France, there is not one single case where the defense attorney is not going to criticize the way in which an admission was given in garde à vue, the way in which an interview was conducted in garde à vue, for the simple fact that nobody was there to verify that all the essential guarantees are enforced. Now that attorneys will be attending the garde à vue, you will get rid of all these arguments in court, so it really is a chance for the police finally to have work which will no longer be criticized in court.
"One of the drawbacks in this reform is that clearly you’ll have a two-tier system where those who can afford a proper defense will have an attorney by their side during all the garde à vue, and others who cannot and who have to rely on the public defense system, maybe they will not enjoy such a full-blown defense. But I hope in fact that things will evolve. And as far as I’m concerned, I registered with a list of volunteers for the weekend with the Paris bar, to attend garde à vue, and I know a lot of attorneys in Paris have done that. There is a lot of good faith and attorneys are really happy to be finally involved in garde à vue, so I’m sure we’ll put a lot of effort into that, and we will not only be looking at the budget."