Officers rushed to the scene at 11.50am following reports of a man armed with a machete suffering a mental health crisis at Skelton Road in Forest Gate.
One of the officers was slashed in the arm as he confronted the man.
He has been taken to hospital but his condition is not life-threatening.
Pictures and videos posted on social media show a huge armed police presence in the area, while the road remains sectioned off with a cordon.
A spokesperson for the Metrolitan Police confirmed that the incident remains ongoing.