Final answer:
The street fight in Act 1 is between the servants of the Capulets and the Montagues, foreshadowing the central feud in Shakespeare's 'Romeo and Juliet'.
Step-by-step explanation:
The street fight in the opening scene of Act 1 is between the servants of two feuding families, the Capulets and the Montagues. This conflict sets the stage for the rest of William Shakespeare's play, reflecting the deep-seated animosity between the two noble families of Verona, which is the backdrop for the tragic love story of Romeo and Juliet.