report it to the police at once ..
car number plates , have a good look at the guys face ( hight , color , bone structure etc ) ..

reporting to the police at once is the best option there is .

scream like crazy , grab a stone , open up couple of heads . Run to the nearest shop or person that u find ( don't show him where u live ) ..

if u get away that day , he will surely be cuming by the next , probably with more friends ..

cops , C.I.D . Guy friends get couple of em follow u from a distance for a few days .. .. .. .. ..

i use to know groups of people who did these kinda harassments just for the fun of it ( cous they were bored or sow U walking down the ally alone couple of times )

try not to walk around alone ...

again

report it to the proper authorities at once ...

( i am sorry sorry sorry sorry for things I was part of back in my adolescence days ( scaring the crap out of chicks walking back home alone ) )