Roberto Firmino scored a hat-trick as rampant Liverpool condemned Claudio Ranieri to a miserable start to life as Watford manager in a one-sided game at Vicarage Road.
The home side had no answer to Liverpool's fluid attacking play.
Sadio Mane became the third African player to score 100 Premier League goals before Roberto Firmino struck either side of the break and Mohamed Salah finished a brilliant individual goal with a smart shot beyond Ben Foster.
Salah's goal takes him level with former Chelsea striker Didier Drogba as the highest-scoring African player in Premier League history with 104 goals.
Firmino completed the scoring in the final minute for his first hat-trick since December 2018.