Stephen Marham, who lives three doors away, said: "I was upstairs talking to my wife. I just heard a massive bang and the house shook.
"My initial reaction was to run downstairs but as I got to the bottom I could see the flats. My partner just screamed 'the flat's gone'."
Video: Dramatic images show house destroyed in Leeds explosionMr Marham said he ran out of the house and saw a man struggling towards him.
"I could see he had been caught up in it," he said. "It was the man who lived in the flat. He was in a bit of a mess so I laid him on the floor in the recovery position.
"I took my jacket off and got a blanket. Another neighbour got one. He's been very, very lucky.
"There were two individuals in the house when it exploded."
He added: "I went into auto-pilot you don't think about the consequences. I knew not to do anything heroic. I could smell gas after it had blown."
A Yorkshire Ambulance Service spokeswoman said three ambulances and the Hazardous Area Response Team had been dispatched.
She said two patients had now been taken to Leeds General Infirmary.