PARIS – The Eiffel Tower stood dark in a symbol of mourning Saturday night as France struggled to absorb the deadliest violence on its soil since World War II: coordinated gun-and-suicide bombing attacks across Paris that left 129 people dead and 352 injured.

President François Hollande vowed that France would wage “merciless” war on the Islamic State group, which claimed responsibility for the mayhem, as investigators raced to track down their accomplices and uncovered possible links to networks in Belgium and Syria.

Paris prosecutor François Molins said three groups of attackers, including seven suicide bombers, carried out the “act of barbarism” that shattered a Parisian Friday night.

He said the attackers in the Bataclan concert hall, where 89 people died, mentioned Syria and Iraq during their rampage. Of the hundreds injured in the six attacks, 99 were in critical condition.

Seven attackers launched gun attacks at Paris cafes, detonated suicide bombs near France’s national stadium and killed hostages inside the concert venue during a show by an American rock band.

“These places are the places we visit every week,” said Ahsan Naeem, a 39-year-old filmmaker who has lived in Paris for seven years. “Streets we walk every day ... All those places will have been full of my people, my friends, my acquaintances.”

Hollande, who declared three days of national mourning and raised the nation’s security to its highest level, called the carnage “an act of war that was prepared, organized, planned from abroad with internal help.”

The president said France would increase its military efforts to crush IS. He said France – which is part of a U.S.-led coalition bombing suspected IS targets in Syria and Iraq and also has troops fighting militants in Africa – “will be merciless toward the barbarians of Islamic State group.”