Muslim groups plan to hold a demonstration outside Paris' main mosque to denounce the "barbarism" of Islamic State militants, as flags fly at half mast after the beheading of a French national.
The demonstration comes as France is engaged in air strikes in Iraq. The government also promised more support for Syrian opposition forces, as the country mourns the murder of hostage Herve Gourdel by jihadists.
President Francois Hollande pledged "determination, composure and vigilance" in the face of threats issued by IS militants sowing terror in Iraq and Syria.
He announced flags nationwide would be flown at half-mast for three days from Friday to mourn the mountaineer murdered in Algeria.
The abduction of Gourdel on Sunday in the restive east of Algeria, and his subsequent execution at the hands of IS-linked Algerian group Jund al-Khilifa, or Soldiers of the Caliphate, sparked global outrage and an outpouring of grief in France.
In an unusual move, the head of the French Muslim Council (CFCM) Dalil Boubakeur called on Muslims to gather outside the Paris Mosque on Friday afternoon to denounce "the horrific and bloody barbarism of the (IS) terrorists".
Earlier this week, the IS group issued a chilling threat to all members of the Iraqi campaign, urging militants to "kill a disbelieving American or European - especially the spiteful and filthy French".
In response, the French presidency announced on Thursday that security in public places and on transport would be strengthened.
The foreign ministry widened its vigilance alert for nationals abroad from around 30 countries earlier this week to some 40 nations - including Indonesia, Malaysia and the Philippines.
Leading French Muslim figures signed a message, run in Friday's Figaro daily, proclaiming solidarity with the victims of IS and proclaiming "We are also 'filthy French'".
