But Khan's remarks had been taken out of context.
The mayor was talking about an increased police presence in the British capital when he said there was 'no reason to be alarmed'.
Khan had earlier released a statement condemning the attacks as 'barbaric' and 'cowardly'.
Trump said on Sunday morning: 'We must stop being politically correct and get down to the business of security for our people. If we don't get smart it will only get worse.'
'At least 7 dead and 48 wounded in terror attack and Mayor of London says there is "no reason to be alarmed!"'
'Do you notice we are not having a gun debate right now? That's because they used knives and a truck!'