Dave Fawbert spotted the boa constrictor, thought to be an abandoned pet, on High Road, Leytonstone, on Saturday morning.
The RSPCA has since taken the reptile to a wildlife centre and is appealing for information about how it may have come to be in the street.
A spokeswoman for the charity said it appeared "someone had found a dead pigeon and then given it to the snake".
Mr Fawbert said the snake was "100% eating the pigeon, it had its jaw open wrapped around it and the head was half-way in already".
He added that he had hoped to see a snake in the wild while on safari in Tanzania a few years ago. "I never did, but I guess now I have," he said.
His Twitter post of the reptile has been shared more than 4,000 times.
Many described the animal as a python, but the RSPCA confirmed it is a boa constrictor.
Mr Fawbert said when he discovered the animal on the pavement between a parked car and a row of shops one passer-by screamed.
But soon a big crowd gathered, he added.
Tags:
news