Leak at Main Shut Off Valve
"Duck" or duct tape is pointless here. You MIGHT get some temporary slowdown from using a self-fusing silicone overwrap, but it's not a solution, and will be difficult to use effectively on an active leak, especially with an elbow involved.

You need to contact your water company and arrange for the water to be shut off from outside the house so this can be fixed correctly. If you get lucky they might be responsible for the pipe up to the shutoff valve.