The US Democrat presidential candidate has attacked President Bush for failing to push for the renewal of a 10-year ban on private use of assault weapons.
The powerful National Rifle Association lobbied against the ban
John Kerry accused his rival of placing gun lobby interests above those of police and gun crime victims' families.
The law expired at midnight on Monday after Republican legislators refused to make time for a vote to extend it.
The move means that ordinary citizens will now be allowed to keep powerful assault weapons in their homes.
President George W Bush has said he supports the ban but he has not pressured Congress to extend the prohibition.
The ban was passed by President Clinton in 1994 after a series of shootings in US schools and fast food restaurants.
Lifting the ban has been a key aim of the powerful, pro-Republican gun lobby and its demise comes just weeks before the US presidential election.
Polls show the majority of Americans support the ban and several police chiefs have expressed concern over its repeal.
Mr Kerry has vowed to make it an election issue.
'Easier for terrorists'
Accepting the backing of the National Association of Police Organisations, he accused Mr Bush of choosing "powerful
and well-connected friends" over police officers and families he had promised to protect.
He added that criminals would now find it easier to get hold of the weapons.
"So, tomorrow for the first time in 10 years when a killer
walks into a gun shop, when a terrorist goes to a gun show
somewhere in America, when they want to purchase an AK-47 or
some other military assault weapon, they're going to hear one
word: 'Sure'", he said.
"George Bush chose to make the job of terrorists easier and make the job of America's police officers harder, and that's just plain wrong."
White House spokesman Scott McClellan described Mr Kerry's words as "another false attack", while earlier the National Rifle Association (NRA) called Mr Kerry "the most anti-gun presidential nominee in United States history".
The BBC's Justin Webb in Washington says Mr Kerry, who is falling behind in the polls, has seized on the issue as a possible vote-winner.
The 1994 ban covered 19 different types of military assault weapons, including AK-47, Kalashnikov and Uzi rifles, as well as high-capacity ammunition magazines holding more than 10 rounds.
In order to get the measure through Congress, Mr Clinton agreed to demands for a vote to be held after 10 years to confirm the ban.
However, Republicans, who now control both houses of Congress, refused to schedule such a vote.