They have also posted an appeal to users to turn in the fanatics responsible for the US terror attacks.
Using the pseudonym Abdullah Ahmed, the MI5 message urges users of the Arabic sites who have any suspicions or evidence to contact the security services.
The appeal stresses the scale of the deaths and the fact that many Muslims were victims.
Some of the preparations for the attacks on the World Trade Center and the Pentagon appear to have been routed through the Web.
Within hours there were 671 responses - 16 in favour of the attacks and 655 containing possible leads.
A senior security source said that MI5 was also monitoring the Web sites and their discussion pages, and joining in to try and flush out information from those who use them regularly.
Anyone with information on terror on the Web should ring 020-7930 9000 in confidence