Sir Peter Fahy, who leads on the Prevent counter-terrorism strategy for the Association of Chief Police Officers, said around 500 Britons had travelled to Syria and Iraq - a higher estimate than the 400 claimed by Foreign Secretary William Hague.
Our estimate at the moment is around 500 people.
There is a huge amount of effort going on both abroad and in this country trying to identify people who may have gone out there to track their movements and, particularly, to make sure that we detain them when they try and get back into this country.
He said "huge amounts of material" was being taken down from the internet every week as part of the effort to stop people being radicalised.
The Greater Manchester Police Chief Constable also warned against "demonising the Muslim community itself" and said:
The vast majority of Muslim people are really worried about this situation and are working with us to try and identify those people that may be at risk.
Leaders of a Cardiff mosque, attended by 3 young men who are now fighting in Syria, say they had nothing to do with their radicalisation.
The Home Secretary has told ITV News that young women have left Britain to fight for the extremists group ISIS in Syria and Iraq.