'The growing body of evidence we are collecting points to the scale being far larger than anyone previously thought.
Mr Kerr said: "We have been shocked by the scale of what we have seen".
"Gangs know they can exploit the most vulnerable in our society and they do because they have realised they can make a profit repeatedly", he said.
Responding to the report, Kevin Hyland, the independent anti-slavery commissioner, said the fight against modern slavery must go beyond arrests, and extend into more convictions as well as improved intelligence gathering and better support for victims.
He said estimates globally suggested 46 million people were victims of modern-day slavery, and gangs were making US$500 trillion a year from exploitation.
"There will be people living and working where victims come into contact with everyone else's so-called normal lives".
"Modern slavery's always been a hidden problem and what's happened since we passed the Modern Slavery Act in 2015 is more and more light is being thrown onto this problem, and rather like child abuse in the past, it's always been there but people were not aware of it, and as they become more aware of it, we uncover the true extent of the problem".
Kerr said signs of slavery included visible injuries, a distressed appearance and any indication they are being controlled by another person.
Kerr said there is a wide age range among victims, and a balance of genders, and highlighted one example of a 12-year-old Roma girl who was brought into Britain to work for a family as a domestic servant.
The PSNI created its human trafficking unit early in 2015, a move Mr Kerr says put them "ahead of the curve".
Speaking to the Evening Standard on Thursday, Mr Hyland said measures to protect other potential victims had not been taken, in a failure he likened to allowing a rapist to "run around London" without police taking action.
Police in Bristol arrested four people last week following a raid at a nail bar in Southmead, Bristol, following a tip off over concerns for a woman.
A 43-year-old woman was arrested at an address in Chesterton Court in Chester on suspicion of modern slavery and a 24-year-old woman and 29-year-old man were arrested on suspicion of modern slavery at an address in High Street in Northwich. Detectives said the woman in question was taken to a place of safety and receiving help agencies with expertise in supporting victims of modern slavery offences.
There has been a wide range of cases of modern slavery and human trafficking uncovered, ranging from a Romanian organised crime gang making €5m (£4.5m) from advertising prostitutes online and laundering the proceeds, to a 12-year-old girl being trafficked into the United Kingdom to take children on a school run.
It said there were 3805 people reported as potential victims in 2016 - an increase from 1745 in 2013.
Aidan McQuade, director of the UK-based Anti-Slavery International, said cuts to police numbers and a general lack of understanding among rank and file officers have been contributing factors in the rise.
"Hopefully the NCA's advertising campaign will lead to more intelligence and greater understanding of the scale of the problem".