"Nothing has been done for years against Islamic fundamentalism… [I call for] firm and strong measures taken immediately to defeat Islamism," Le Pen, the leader of the third-largest political party in France, said in a statement, cited by BFMTV.
A 30-year-old suspect, already on the authorities' radar, has been detained on the premises of the Seveso chemical manufacturing plant in the Saint-Quentin-Fallavier commune of the Isere department in southeastern France.
Investigators found a decapitated body and a severed head nearby, with an Islamist flag near the former and Arabic inscriptions near the latter.
France was targeted by extremists earlier this year when 12 people were killed in the Paris offices of the Charlie Hebdo magazine. The country has been on high alert ever since.
Defense Minister Jean-Yves Le Drian announced an extraordinary Defense Council meeting at 13:00 GMT. Hollande canceled his trip to Brussels and is expected to take part in the session.