Are they in your house or any sort of building?
You need to get rid of them quickly and trapping takes too long. They breed like rabbits so the longer you take to kill them the more there will be to kill.
If you have neighbours you really should get the pest control in and get the problem sorted now.
The Prevention of Damage by Pests Act 1949 places an obligation on owners or occupiers to control rats and mice on their property and to inform the local Council of infestations.
"Local councils also have enforcement powers to ensure that rodent control is carried out. If necessary enforcement notices can be served on the persons responsible for any land that is infested requiring that they are eradicated. If these notices are not complied with then powers are available to either carry out the works required directly and charge the costs or to take a court case."
http://www.hounslow.gov.uk/rats_pest_control
Trapping only works if you have one or two rats. If you still have rats after two or three weeks you definitely should use poison.