The attack on the youth has caused alarm among senior officials in France.
He was taken from his camp by a gang of masked men said to have suspected him of taking part in a burglary. They then beat him unconscious and left him for dead in what is being seen as a vigilante attack.
He was later found dumped in a supermarket trolley and was last night said to be in critical condition in hospital.
The teenager, named by Le Monde newspaper as Darius, had been living with his family in the makeshift camp close to the Cité des Poetes housing estate.
The 200 or so people in the camp fled the area the same evening, it was reported.
Mr Hollande called the attack “unspeakable and unjustifiable”.
In a statement yesterday, he said “all efforts should be made to find the perpetrators”.
Luc Poignant, a police union official, told French television in an interview that about a dozen people had forced their way into the Roma camp on Friday night after they learned an apartment nearby had been broken into.
The camp is in the suburb of Pierrefitte-sur-Seine north of Paris.
Reports said police officers found the young victim abandoned by the side of a road.
Mr Poignant said the teenager was taken to hospital where he was put into a medically induced coma “because he was in so much pain”.
Interior minister Bernard Cazeneuve, in charge of French police, also condemned the vigilantes.
“It is exclusively up to security forces to ensure that public order is respected,” he said in a statement.
Anti-racism groups say violence in France is rising against Roma migrants who come primarily from eastern Europe and are often blamed for petty crime.
The presence in France of thousands of Roma people from eastern Europe has been a vexed political issue since the summer of 2010 when the conservative president of the day, Nicolas Sarkozy, called for the removal of their illegal camps, saying they were hotbeds of crime.
Since coming to power in 2012, the Socialists have stepped up the policy of camp clearances and France has also continued to deport thousands of foreign camp-dwellers every year, mainly to Romania.
Human rights organisations have warned against a rise in violence against the Roma in France, where they are often accused of fuelling a wave of petty crime.
The mayor of Pierrefitte-sur-Seine said Darius had been questioned by police several times this month in connection with a string of robberies in the housing estate.
After the attack, Roma in the camp fled “with what they could carry on foot, by tram or car”, a Romanian garage owner told one newspaper.