Le Pen's National Front (FN) said they were turned away because of a lack of space at the venue, a dance hall in Vincennes, just east of Paris.
Le Monde, Liberation and L'Humanite newspapers said they would boycott the event out of "solidarity" with the other outlets.
Buzzfeed and Mediapart were among the online news sites that said on Twitter they were refused accreditation for the event, as well as reporters from Britain's Sky News, the US monthly The Atlantic and the Italian public TV channel Rai.
Le Pen's rival, centrist Emmanuel Macron, is favourite to clinch the presidency today with the last opinion polls before the vote giving him a lead of more than 20 points.