Lawmakers and advocates on Wednesday called for safeguards to be placed on the domestic use of drones in order to protect the privacy of Americans.

Congress has ordered the Federal Aviation Administration to open up airspace to the unmanned aircraft by October 2015, a decision expected to see thousands of drones criss-crossing the sky within a few years.

Their imminent proliferation has stirred a debate, amid concerns they may be deployed to snoop on law-abiding citizens.

"We have to prevent drones from becoming alternatives to police patrols," Amie Stepanovich of the Electronic Privacy Information Center told the Senate Judiciary committee during a hearing on the matter.

US police departments have already expressed interest in drones to monitor protests, possibly with nonlethal arms, she said.

According to Senator Patrick Leahy, a Democrat who heads the committee, said the domestic use of surveillance drones would have a "broad and significant impact on the everyday lives of millions of Americans."

As he spoke, he showed the hearing a small black drone weighing just one kilogram (2.2 pounds).

"When is it appropriate for law enforcement to use a drone, and for what purposes?" he asked.

Through the end of the decade, some 30,000 drones for civilian use could fly in US skies, Leahy said, citing the FAA.

Senator Chuck Grassley, the top Republican on the committee, said drones "carry sophisticated technology ... limited only by imagination," and therefore creating "greater responsibility."

A drone is "cheaper" to mobilize than a helicopter for finding a missing person or to take pictures of a crime scene, said Benjamin Miller, unmanned aircraft program manager at the Mesa County Sheriff's Office in Colorado.

However, Miller said he was in favor of the use of judicial warrants when the drone activity touches on private life -- except in emergency situations.

If nothing is done to "update" the legislation on privacy, "Americans will reject this technology," said Ryan Calo, assistant professor at the University of Washington School of Law.