Democratic Party attorney generals of Connecticut, Maryland, Massachusetts, New Jersey, New York, Pennsylvania, Oregon, Washington and the District of Columbia have joined the petition, to be filed in federal court.

The suit will seek a temporary restraining order nationwide and argue that the decision violates states' rights to regulate firearms, officials said.

"This decision is unconstitutional. It is unlawful. Frankly it is terrifying," said Bob Ferguson, Washington state attorney general, taking the lead on the suit to be filed in Seattle.

"This unprecedented move is not only disastrous for public safety but undermines our state laws meant to keep firearms out of the hands of dangerous individuals."

After a lengthy legal battle, the government reached agreement last month with Cody Wilson, a militant gun rights advocate from Texas.

He successfully argued that the US Constitution's Second Amendment, which guarantees the right to private gun ownership, should extend to a person's right to make guns at home -- uncontrolled by authorities, since they will bear no serial number.

It means anyone with a 3D printer -- which costs around $2,000 and can be programmed to build objects of almost any shape -- will be able starting next week to make plastic-bodied guns at home for just a few hundred dollars each.

Security experts fear that the guns could evade metal detectors used in public buildings and airports.

But gun enthusiasts say that without some metal parts, the guns will be unreliable -- and might even explode in a user's face.

Democratic lawmakers have decried the settlement and demanded an explanation from the Republican administration, which has supported gun owners' rights.