The legal basis that allows a Schengen State to restrict the freedom of movement in the Schengen area at its internal borders is article 2.2 of the Schengen Convention.

It says that, where public policy or national security so require, a Contracting Party may, after consulting the other Contracting Parties, decide that for a limited period national border checks appropriate to the situation shall be carried out at internal borders. If public policy or national security require immediate action, the Contracting Party concerned shall take the necessary measures and at the earliest opportunity shall inform the other Contracting Parties thereof.

Martin
MediaVisa.net