A colleague found Raymonde Talbot's body in her office on the Maport city's main business street on Friday, police said.
The third-floor office was locked and no murder weapon was found, they said.
"It's blood-chilling," a fellow lawyer said.
"We don't know the exact circumstances but the whole profession is concerned."
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Marseille deputy mayor Caroline Pozmentier said the authorities were "revolted by this odious act which has rocked not only the legal profession but the entire city."
While little is known about the circumstances surrounding this murder, there has been growing concern in government circles at a wave of killings in Marseille.
So far this year, 18 people have been killed there in crimes linked to inter-gang rivalry, many of them gunned down in killings linked to the drugs trade.
On September 6th, Prime Minister Jean-Marc Ayrault announced beefed-up security with 250 extra police to tackle deadly gang violence in Marseille.
But a number of police officers have been also suspended as part of a corruption investigation in the city.