Allen and Christian Benteke, who have struggled to secure first-team spots this season, combined well for the opener after 35 minutes when the Welshman burst on to the striker's knockdown.

Klopp's side continued to dominate after the break and Firmino added a second with a right-foot shot from outside the box after 76 minutes.

The victory left Europa League finalists Liverpool eighth in the table with two games remaining. Watford stayed 13th.