Suspected Islamist militants targeted in a raid on the Paris suburb of Saint Denis on Wednesday morning were planning a fresh attack on the city's La Defense business district for the following day, three sources told the Reuters news agency.
Eight people were arrested in the seven-hour siege, during which 5,000 rounds were fired by police.
The number of people killed is not yet confirmed, but is believed to be at least two - including one woman thought to have activated a suicide vest.
The alleged foiled plot would have targeted a district which holds some of France's biggest companies, including oil firm Total and the bank Societe Generale.
According to the sources, the attacks would have taken place at the Quatre Temps shopping centre and the main square of the district.
In a press conference, Paris prosecutor Francois Molins confirmed that the terror cell flushed out in the raid was planning "new attacks", but gave no further details.
Hours after the raid, a loud blast, believed to be a controlled explosion, was heard at the flat - suggesting explosives were being held there.
The primary target of the raid was Abdelhamid Abaaoud, a Belgian Islamist believed to have been the ringleader of Friday's massacre, which killed 129 people.
Officials have yet to formally identify those arrested and killed in Wednesday's siege, but said Abaaou was not among those detained.