Hi farhan
What an interesting question. My online seaches threw up many hate sites, so be aware of this when doing research for yourself.
Jews were 'noticeable' to the Roman authorities because they refused to recognise the emperor-cult, and because of their fierce resistance to Roman advances into Palestine in the frst century.
Emperor worship was a simple civic ceremony that involved publicly worshipping the divine emperors, by placing a pinch of incense on their altar. Christians were noticeable for the same reason and in fact the two were not distinguished by the Romans for a long time, and persecution by a number of pagan emperors hit both groups from time to time.
In the 3rd century the Emperor Caracalla allowed Jews to become Roman Citizens :
http://www.spiritus-temporis.com/ashkenazi/ori gin-of-ashkenazim.html
But the arrival of Christianity as the official state religion, in the time of Constantine marked a change
http://vlib.iue.it/carrie/texts/carrie_books/s eaver/text.html and it seems to be from this time onwards that sporadic outbursts of state persecution break out. Constantine set the ball rolling by passing laws such as forbidding Jews to have Christian slaves, and forbidding Jews entry into Jerusalem.