If you witness an arrest...

Try to get the name of the person being arrested and the reason the police have given for the arrest.
Get the ID number of the arresting officers and others involved.
Ask what police station the they are being taken to.
Look around for other witnesses and get their contact details.
Especially look around for people with cameras who may have footage/photos of the arrest or events leading up to the arrest. Get their contact details!
Briefly note down what happened and what led up to the arrest (do a more complete statement when you get home). Don't forget to note down when and where it happened.
Contact arrestee support and pass on the details you have collected.

If you witness an assault, most of the above still applies.

If you videoed or photographed an incident.
Be very wary of police around you, they may try to get the camera off you, perhaps even arrest you to do so.
If safe to do so, find out the name of the person arrested or assaulted.
Get together other witnesses, their testimony is vital to put your images into context so get their contact details.
Please DONT upload the video to youtube at this stage!
Try to locate a legal observer (orange bib with words 'legal observer') or call us 07946 541 511 as soon as possible.

Look after the footage, make sure the tape/memory disk is labelled and not accidentally wiped or mislaid.

Please contact GBCLegal as soon as possible either by phone 07946 541 511or email gbclegal[at]riseup.net so that we can make sure any witnesses are put in touch with solicitors defending those arrested or assaulted or put you in touch with the person affected.