They made the chilling threats on Twitter after at least 129 people were killed by a series of co-ordinated terror attacks on Paris on Friday night.
After gloating about the horror that shocked the world, armchair jihadists tweeted that London could be next.
The twisted tweeters also claimed to have two other major capitals in their sights – Washington DC and Rome.
French medics evacuate an injured man near to the Bataclan Theater
London was last hit by a terror attack in July 2005, when 52 commuters were killed by suicide bombers linked to Islamist group al-Qaeda.
But officials have reportedly been working tirelessly to thwart ISIS attacks since the militants declared a caliphate last summer.
Last month Andrew Parker, the head of MI5, admitted the security service can "never be confident" in stopping all terror plots against Britain.
Armchair jihadists gloated about the attacks on Twitter
Washington DC's police department has deployed extra officers to high-profile locations in the city – including the government's Capitol complex.
But aspokeswoman insisted: "There is currently no known threat to the Capitol complex."
Security has also been beefed up at the city's French Embassy but a fashion show went ahead as scheduled at the venue on Saturday.
Hosts paused the event for a moment of silence to honour the tragic victims of the Paris attacks.
French president Francois Hollande at the Stade de France
The last terrorist incident in Washington DC was in April 2013, when a man sent letters laced with ricin to Barack Obama.
James Dutschke's plot to poison the President was thwarted by officials and he was sentenced to 25 years in prison.