Paris: Francois Hollande blamed the Islamic State group for orchestrating the deadliest attacks inflicted on France since World War II and vowed Saturday to strike back without mercy at what he called ‘an act of war.’

Hollande said at least 127 people died Friday night in shootings at Paris cafes, suicide bombings near France's national stadium and a hostage-taking slaughter inside a concert hall.

Speaking after an emergency security meeting to plan his government's response, Hollande declared three days of national mourning and raised the nation's security to its highest level.

Hollande blamed the carnage on what he called ‘a terrorist army, the Islamic State group, a jihadist army, against France, against the values that we defend everywhere in the world, against what we are: a free country that means something to the whole planet.’

As he spoke, French anti-terror police worked to identify potential accomplices to the attackers known to have committed the attacks.

The perpetrators, at least in public, remained a mystery: their nationalities, their motives, even their exact number. Authorities said eight died, seven in suicide bombings, a new terror tactic in France. Police said they shot and killed the other assailant.

World leaders united in sympathy and indignation, New York police increased security measures, and people worldwide reached out to friends and loved ones in France.

The violence raised questions about security for the millions of tourists who come to Paris and for world events routinely hosted in the normally luminous capital, where troops were deployed to support police trying to restore order.

One of Europe's most heavily visited tourist attractions, the Disneyland theme park east of the capital, announced it would not open for business Saturday, a rarity.

Prosecutor's office spokeswoman Agnes Thibault-Lecuivre said authorities couldn't rule out the possibility that other militants involved in the attack remained at large.

Hollande said France - which is already bombing IS targets in Syria and Iraq as part of the US-led coalition, and has troops fighting militants in Africa - ‘will be merciless toward the barbarians of Islamic State group.’

‘It's an act of war that was prepared, organized, planned from abroad, with internal help,’ he said.

Reflecting fears in other European capitals of the risk of coordinated or copycat attacks, the British government scheduled its own emergency COBRA intelligence committee overseen by Prime Minister David Cameron. Italy said it, too, was raising security levels on borders and major public places.