"Not only are criminals buying guns online, they are doing so openly and brazenly by advertising that they want to buy them," he said.
A four-month investigation by Bloomberg's group Mayor's Against Illegal Guns found that 1 in 30 of those openly advertising to buy firearms had felony records.
"Their history includes charges of illegal possession of guns, domestic assaults, protection orders and robberies with dangerous weapons," John Feinblatt, Bloomberg's policy advisor, said.
In 2011, a jilted ex-boyfriend in Illinois with a criminal record took to the ArmsList website to skirt a background check and purchase the gun he used to kill Jitka Vesel in a Chicago suburb.
More than 10 years ago, Eyewitness News investigations exposed how the anonymity of the internet allowed the purchase of assault rifles without a background check. We picked up them up at the home of the private seller or hand the weapon delivered to a nearby post office box.
Since then the number of online gun sites has exploded. The Mayor's investigation estimates that on one website alone some 25-thousand guns may have been purchased by criminals.
"The big surprise to me was that people with criminal records would have the audacity to put their phone numbers and email address - they just go on the record if you will. They must feel there's no enforcement," Bloomberg said.
The investigation did not check those looking to sells guns and was limited to Armslist.com. It could not determine how many, if any, of the prospective felon buyers actually purchased weapons.
Bloomberg said the findings, which come in the wake of the Washington Navy Yard shooting that left 12 dead, were further evidence that Congress needed to pass legislation requiring background checks for all gun sales, including private sales where no such check is presently required.
"We know what happened this week has happened before - and sadly it looks like it will happen again until we get serious about this issue of just too many guns around," Bloomberg said in his first remarks about the Washington shooting.
ArmsList.com has disclaimers on the site urging users to follow the law and to report illegal activity.
Some gun rights groups were angered by Bloomberg's surveillance of the online forum, which is largely used by legal buyers and sellers.
"I think that's creepy and disturbing," said Erich Pratt, director of communications for Gun Owners of America. "He's a petty little tyrant who wants to ban big sugary drinks and restrict the types of guns people can use to defend themselves."
Pratt also rejected Bloomberg's calls for more gun regulations.
"What needs to happen is less regulation," he said. "We need to end gun-free zones at schools and military bases that make people sitting ducks."
The Navy Yard shooter, Aaron Alexis, passed a background check at a Virginia gun shop before he bought the shotgun he used in his rampage.
A request for a comment from Arms-list.com was not responded to.
Gun Advocate groups argue that background checks of private sales would violate constitutional rights of law-abiding people while pushing criminals to get weapons on the black market.
Some information from The Associated Press