She hit out at the government's record on crime on the same day that interior minister Claude Guéant was in the city himself to install a new senior official, Alain Gardère, to combat crime in the region.
"The crime policies of Nicolas Sarkozy have been a fiasco," she said at a press conference.
"The president has been in charge of security for ten years," she said, referring to the period Sarkozy spent as interior minister before he became president in 2007. "He has failed."
Attacking the nomination of Gardère, who is the third person to hold the crime supremo role in two years, she said "we don't resolve crime problems by having a waltz of officials."
Interior minister Guéant visited the tough district of Porte d'Aix himself on Monday. The area became well-known as the location of a notorious car park that was abandoned by its owners after it was taken over by gangs of youths in the summer. He pledged a reinforcement of 360 additional police officers for the city, including 200 from the anti-riot CRS squad.
"We've been abandoned for years," one shop owner in the area told AFP on Monday.
Aubry claimed she would present proposals that genuinely combat crime. "I have come here today to say quite clearly that I will be the president for the security of all French people," she said.
By taking on the crime issue she hopes to tackle President Sarkozy on a topic that has been one of his core themes since the 2007 election campaign.
According to left-wing newspaper Libération, Aubry is also trying to seize the initiative on an issue that has been "explosive" in the Socialist party.
"To talk about security means being accused of playing the right-wing game," said the newspaper on Tuesday.
"The Socialist party has everything to gain by taking on the battle against crime," François Miquet-Marty of the polling institute Viavoice told the newspaper. "It resonates very strongly with working class people who are the main victims of crime."
To back up her visit, Aubry promised on Monday to reinstate over 10,000 police officers who, she claimed, have been lost since Sarkozy took office.
Aubry's visit is also designed to help her catch up the current front-runner in the battle for the presidential nomination, François Hollande, who is ten points ahead of her according to the most recent poll.