Know Before You Go
With 9$ android devices readily available you can not only have a burner for phone calls but also for data. Apps like this https://f-droid.org/app/es.esy.CosyDVR might comein handy. Also a local file share for the occasion. 
With the proliferation of personal technologies, protesters of all political persuasions are increasingly documenting their protests—and encounters with the police—using electronic devices like cameras and mobile phones. In some cases, getting that one shot of the riot police coming right at you posted somewhere on the Internet is an exceptionally powerful act and can draw vital attention to your cause.