And further goals after half-time from Saka and Declan Rice compounded the home side's misery.
It leaves Moyes without a win in five league games having previously beaten Arsenal twice this season - 3-1 in the Carabao Cup and 2-0 in the league.
'A really poor day for us,' said Moyes, who returned to the club in December 2019. 'Very difficult to explain, really. Arsenal played really well. (It's) difficult to explain how we played so poorly.
Wesy Ham suffered a 6-0 humiliation by London rivals Arsenal at the London Stadium
David Moyes says it was his side's worst defensive performance since his return to the club
'Disappointing thing for me, I don't think since I've come back to the club my teams (have) defended that way, ever.
'We were weak today. We didn't do the jobs well enough. We didn't fight to contain it harder and make sure we didn't concede. We could have conceded other goals as well.
'Arsenal went up another level today if I'm being honest. I saw that in them. Maybe they had something to prove as well, coming here and getting a result. But I can't only talk about us, and we weren't at the races at all.'
The performance saw home supporters leave at half-time. Though Moyes was circumspect, citing the good times Hammers' fans have enjoyed in recent years. That includes winning the Europa Conference League last season.
He added: 'To be a football supporter, especially this club who people know a lot more about than me, I came back twice to keep them out of relegation - there is always going to be bad times.
Moyes was also full of praise for Arsenal, claiming the Gunners 'went up another level'
'It was only three months ago we were having probably the best time West Ham has ever had.
'Let's be fair, probably the last three years has probably been as good a time as West Ham has had. Sixth, seventh in the league, semi-final of Europe. A final of a European competition. I still think that this club has grown.
'Undoubtedly we've had a terrible day today so I understand totally them leaving. Sometimes football clubs are going to have a bad day. We had a bad day. But I certainly won't forget the good ones.'
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