Mobile wifi hotspots are prolific but they are often expensive without contract, limited in their number of simultaneous connections, range and battery life. Below, a recipe for a mobile internet system that's low-cost, able to serve a large group, yet still portable enough to be carried via backpack or bicycle.
The idea has been successfully field tested at the OccupySF camp in Justin Herman Plaza, a backpack version has been brought to several actions.
With the exception of a 4G modem, you can usually scavenge other necessary parts.
 What you need
- Clear or other 4G Desktop Modem. Series M & G work fine. $10-20 Used. (Try Amazon Ebay, or Craigslist).
- Wifi Router / Access Point. Ubiquiti Picostation, Linksys WRT54G, anything that takes 12V should be okay!
- Power connectors, wire, battery clips, a diode and fuse
- 12V Battery
 Basic build
- Splice two power connectors or (if supported) a passive power-over-ethernet cable together
- Add reverse polarity protection - A fuse in series with your circuit & a diode in parallel (read about power protection circuits)
- Connect leads to battery clips
- Connect Clear modem to Wifi AP
 Notes on Clear
Clear customer service sticks to a script, out of the ordinary requests will result in you being transferred from person to person and possibly denied. If you find yourself stuck, or they won't activate your used modem message @clear on Twitter --- reps there provide fast, competent resolution.
Regular unlimited service is ~$50/month, they also offer mostly unadvertised $10/day, $25/week passes, which may be more economical if you are only using the device for events.