The visitors were swept aside in an incredible opening 18 minutes during which the champions scored five times.
Three more were added after the break, with Bernardo Silva grabbing his first hat-trick for the Blues and Kevin De Bruyne's late thunderbolt was arguably the pick of the bunch on a thoroughly enjoyable day at the Etihad - and one that won't be forgotten in a hurry.
Breathless, irresistible, scarcely believable… City ripped Watford to shreds with the sort of blistering start that may never be witnessed by the Premier League for a very long time.
If the champions were still angry at last weekend’s defeat to Norwich, it was Watford who paid a heavy price.
With 18 minutes of this game gone, the Blues had scored five times. Five times.
It was mesmerising football that the short-sleeved capacity crowd lapped up in the early autumn sunshine.
Where to begin?
And the celebrations had barely died down as Watford came within a whisker of equalising through Gerard Deulofeu’s clever run and low shot that was palmed around the post by Ederson.
Aguero stepped up to stroke home his eighth of the season and his 100th Etihad goal – a feat only achieved by two other players in the Premier League era.
100 - Sergio Aguero has scored his 100th Premier League goal at the Etihad - he is one of just three players in the competition's history to net 100+ goals at a single venue, alongside Thierry Henry at Highbury (114) and Wayne Rooney at Old Trafford (101). Milestone. pic.twitter.com/1CaVQqMNzd— OptaJoe (@OptaJoe) September 21, 2019
The third, on 12 minutes, was all about Mahrez who danced past a number of challenges before being felled on the edge of the box – he took the resulting free-kick which deflected in off Tom Cleverley.
And the goals kept coming.
With Watford’s porous defence looking like they might concede with every attack, De Bruyne whipped in a corner that was nodded across goal by Nicolas Otamendi and headed home by Bernardo on 15 minutes.
Three minutes later, Otamendi slid home Aguero’s low cross to make it five.
It was ruthless attacking football by the rampant Blues, and it was quite a way to mark the Club’s 125th anniversary celebration.
There were further opportunities, but for the final 27 minutes or so, Watford rode their luck and the Blues eased off the gas a little.
But the ceasefire ended just three minutes after the re-start as persistence by David Silva and Bernardo eventually ended with an opportunity for the Portuguese play-maker who neatly gathered the ball before slotting it past Foster for his second of the afternoon.
It was no more than KDB deserved. He was brilliant from start to finish.
1' ⚽️ @21LVA— Manchester City (@ManCity) September 21, 2019
7' ⚽️ @aguerosergiokun (PEN)
12' ⚽️ @Mahrez22
15' ⚽️ @BernardoCSilva
18' ⚽️ @Notamendi30
48' ⚽️ @BernardoCSilva
60' ⚽️ @BernardoCSilva
85' ⚽️ @DeBruyneKev
You'll do for us, lads.
🔵 8-0 🐝 #MCIWAT #ManCity pic.twitter.com/0G6Bx81LEa
City punish Watford. Again.
This was our twelfth successive win against the Hornets and whatever they've done to upset this City side, it must be bad!
It's now 33 goals in seven matches against Watford.
Aguero has scored 11 in eight matches against them and, of course, the Blues struck six against Watford at Wembley in the FA Cup last May..
Four of those goals came in the last 29 minutes, which, added to the five in the first 18 minutes of this game means the Blues bagged nine goals in just 47 minutes of football against the Hertfordshire side.
Two more after half-time made it 11 in just over an hour of football.
Star man: Bernardo Silva
Three goals and a typically industrious and mesmeric performance by the fan favourite.
What it means…
City reduce the gap on leaders Liverpool to just two point ahead of the Merseysiders’ Sunday afternoon trip to Chelsea.
City begin the defence of the Carabao Cup on Tuesday evening with the short trip north to face Preston North End.
Then, next Saturday, the Blues take on Everton at Goodison Park (5.30pm kick-off).