A gunman reportedly shouted "it's for Syria" and "Allahu Akbar" – the Arabic for "God is great" – before slaughtering around 100 music fans at a concert hall.
The attacks came just hours after British ISIS butcher Jihadi John was said to have been killed by a US drone strike in Syria. Several ISIS supporters celebrated the horror attacks using the sick hashtag 'ParisIsBurning'.
One said: "God is great and thank God for these lone wolf attacks. At least 100 hostages and countless wounded."
His tweet was sent from the Kuwait port of Mina Abdulla, according to Twitter's location settings.
Another added: "Oh God, burn Paris as you burned the Muslims in Mali, Africa, Iraq, Syria, and Palestine."
Some fans even boasted London, Rome and Washington DC would hit by the next horror attacks.
But some social media users used the hashtag to share their sympathy with those hurt and killed in the attacks.
Cameron Dylong said: "Prayers up to Paris right now."
And Armani Bradley wrote: "Disgusting how ISIS Twitter accounts are hashtagging 'Paris is burning'.
"Celebrating the deaths of innocent people, what a vile world we live in." "The terrorists who were not far from here were killed," Hollande said on television after visiting a security command centre near the Bataclan concert hall where the gunmen held people hostage before an assault by security forces. "It was a bloodbath," Julien Pearce, a reporter for France's Europe 1 radio station, told CNN.
"People yelled, screamed and everybody lying on the floor, and it lasted for 10 minutes, 10 minutes, 10 horrific minutes where everybody was on the floor covering their head(s)."
"We heard so many gunshots and the terrorists were very calm, very determined and they reloaded three or four times their weapons and they didn't shout anything. They didn't say anything," French radio reporter who was inside the Bataclan theatre recounted.
In January, ISIS praised two gunmen who killed 12 people at the Paris offices of the satirical magazine Charlie Hebdo.
The terror group called them "heroic jihadists", according to the Site Intelligence Group.
In June, a suspected Islamist decapitated a man in a gas factory in the south-eastern city of Lyon.
Two months later three tourists overpowered a heavily armed gunman linked to ISIS on a Paris-bound train.