Gas engineer Tony Smith, 46, spotted the four foot long rat while working at a block of flats Hackney Downs.
The rodent, which was found dead, was discovered near to the play area which backs onto a railway track on Thursday.
Mr Smith snapped a picture of the rat being held by electrician friend James Green, 46, to the glee and disgust of his friends.
He said: “This is the largest rat I’ve ever seen in my entire life.
“I’ve got a cat and a Jack Russell and it was bigger than both of those put together.
“I’d say it was about four foot.
“We were going to stick it in the bin, but before we did we thought we better take a picture of people won’t believe it’s real.”
Vermin Seen Around London
He said: “The bins get left open nearby so these little fellas have a pretty good diet.
“They eat fried chicken and rice all the time.”