The incident happened in front of a house that is rented to two famlies on 222nd Street in Springfield Gardens.
Police say the package was sitting on the front stoop for a number of days.
The landlord, a 73-year-old man, picked up the package, and it exploded.
"I heard boom and this big explosion, thought it was an accident," a resident said.
The package appeared to be a cylinder, similar in shape and size to a Quaker Oats container.
"It was way too loud and it vibrated," a resident said.
Police say there was no evidence of shrapnel, just black powder.
"I saw a man lying in the grass he was actually on his knees and he was on fire," said Rick Thomas, a neighbor. "He was screaming out in pain, a lot of pain, he was engulfed, the flames engulfed his whole top half."
Neighbors quickly rushed to help the man covered in flames as first responders rushed to the scene.
The victim is said to have suffered second and third degree burns over 80-percent of his body, and is currently in critical condition at Nassau University Medical Center.
"When I got out there, there was a neighbor across the street they had ran to aid him and they put towels around him to put the fire out, and someone called 911, and the firemen were on the scene first," a neighbor said.
Investigators have yet to identify the name written on the mysterious package.
The U.S. Postal Service released a statement:
"This matter is currently under investigation with law enforcement and we cannot provide further comment at this time."
The victim owns the home but does not live there. It is currently rented out by two families.
The investigation is ongoing.