Hundreds of people gathered in Paris Friday answering a call by Muslim leaders to denounce the "barbarism" of Islamic State militants, as flags across France flew at half mast after the beheading of a French national.

Imams and ordinary Muslims have rallied to condemn the execution of 55-year-old mountaineer Herve Gourdel this week by militants in Algeria with ties to the IS militants, as the country mourned the brutal murder.

"We French Muslims say stop to barbarism, stop to terrorism," Dalil Boubakeur, head of the French Council of the Muslim Faith -- an official representative for the country's roughly five million Muslims -- told the gathering outside Paris's main mosque.

He said the demonstration was a "vibrant expression of our desire for national unity and of our unwavering will to live together".

"Islam is a religion of peace", he said, adding it "orders respect for life".

Leading Muslim figures also signed a message published in French newspapers Friday condemning "atrocities committed in the name of a murderous ideology hiding behind the Islamic religion".

"We are also the 'filthy French'," said the statement, referring to an epithet the Islamic State group has used.

Flags at half-mast

Gourdel was kidnapped on Sunday by Algerian group Jund al-Khilifa shortly after a chilling call by IS militants for Muslims to kill citizens from countries involved in a US-led coalition fighting the extremists, "especially the spiteful and filthy French".

The threat sparked concern in France, and authorities have since strengthened security in public places and on transport.

The foreign ministry also widened its vigilance alert for nationals abroad from around 30 countries earlier this week to some 40 nations -- including Indonesia, Malaysia and the Philippines.

France has said it refuses to be cowed by IS threats. The government announced Thursday air strikes had been carried out in Iraq -- the second bombing raids in the space of a week.

Fighter planes destroyed four hangars near Fallujah city, west of Baghdad, that "likely" contained weapons and other equipment, a military source said.

Paris opposed the 2003 invasion of Iraq but was one of the first to sign up for an active role in the campaign against the IS group.

It has six Rafale fighter jets based in the United Arab Emirates, carrying out the strikes in Iraq.

While it has pledged to push forward with the air strikes, Paris has stressed it will not deploy ground troops, nor will it expand operations to Syria, as the United States has done.

But the presidency announced France would "continue and intensify its support for Syrian opposition forces who are fighting militant groups".

'Overwhelming passion'

Back in the southern Alpine village of Saint-Martin-Vesubie, where Gourdel worked as a mountain guide, locals, school students and other guides marched Thursday evening in memory of a man they described as hugely curious about the world.

More than 800 people -- including three imams -- took part in the march past his office, stopping at times to light candles or lay flowers down.

"Herve was a humanist full of an overwhelming passion for the mountain that illuminated his life," fellow guides said in a joint statement.

The mountaineer was kidnapped while hiking in a national park in eastern Algeria that was once a magnet for tourists but later became a sanctuary for Islamists.

Four Algerians who accompanied him on the trek were detained by local authorities who are questioning whether they may have played a role in the abduction.

Flags throughout France will fly at half-mast until Sunday, and authorities in Gourdel's home town of Nice are planning a gathering in his memory Saturday.

Other anti-racism associations, unions and political parties are also calling for a gathering in Paris Sunday.