The gunman, who is in his 30s, holed up inside a classroom as tactical police began to surround the Charles Perrault school in Vitry-sur-Seine in the Val de Marne.
"Negotiations have begun and we are trying to get the man to give himself up," said an officer at the scene.
"The hostage is believed to be the parent of a child. Other people were briefly held, but they were quickly released. Special forces police are on the site."
The Charles Perrault school on Rue Edouard Til in Vitry-sur-Seine
The drama began soon after 7.30am as children began to arrive for a summer camp along with teachers and other parents.
Nobody is believed to have been hurt in the drama, and no shots have been fired. A security perimeter has been established by the RAID special forces.
The school, which teaches children aged three to 12, is closed for lessons for the holidays, but holds a number of summer camps.
