1.Firstly dont try what Pritam has suggested..it can be dangerous for u and ur dog especially when ur dog is friendly types.
2.Second whether it works or not is a different issue but u do ur bit of taking legal help, municipal authorities, police, animal welfare..use all options.If something goes wrong u shouldnt be blamed at a later stage.
3.It is ur responsiblity to protect ur dog.Walk with a companion for a few days so that the single dog realises that u r a pack and not alone.
4. Carry a heavy stick while on walks, u have all the right to protect urself and ur dog if the concerned authorities fail to take action.Dont hesitate to use it if attacked.
5.Think of the dog as ur baby, wife, lil bro or sister..wat wud u do if a dog or person attacked them...thats what u have to do here aswell.Will u ask them to stay indoors, will u complain to the authorities or not, will u use force if necessary or not.
6.If u are helpless even after this, put ur dog for adoption..You dont deserve him and he deserves a proper pack leader who can protect him