French RTL radio reports the suspect is known to police as a petty criminal.
BFM TV reported the hostage-taking isn't related to terrorism, which was behind last week's separate hostage-takings in the Paris area. BFM said the hostage-taker, reported to be armed with a military-style weapon, has already contacted police himself.
Le Monde and BFM both said two people were being held hostage.
Police cordoned off an area around the post office, with a civil defence helicopter hovering.