Dave Fawbert spotted the snake, which he believed at the time to be a python, whilst walking home from the bakers along Leytonstone High Road on Saturday.
His picture of the snake has been retweeted more than 4500 times.
Fawbert, editor of shortlist.com , wrote on the magazine's website: "I was reminded that, on a safari trip to Tanzania in 2011, I spent a large proportion of time desperately hoping to see a snake in the wild and had no luck. Turns out I just needed to go walking in Leytonstone instead."
RSPCA inspector Rebecca Bedson said: "We received a call yesterday morning reporting a stray snake in High Road, Leytonstone. I arrived to find the boa constrictor in a box and transported him to a wildlife centre where he is currently being assessed.
The boa constrictor was seen eating the pigeon in Leytonstone, east London.
"I'm very keen to find out how he came to be in such a dangerous situation. Exposed like that on a street could have meant anything might have happened to him, he could have been run over by a car or attacked by another animal.
"It might be that he is an escaped pet, or more worryingly, someone may have deliberately dumped him and left him to fend for himself. Either way, anybody with any information can give me a call in complete confidence on our inspector's appeal line by phoning 0300 123 8018."
The charity said that it is encountering widespread neglect of reptiles as many people are unaware of how much of a commitment these animals are when they take them on. As a result they now rescue hundreds each year.
