The FAA Reauthorization Act, which President Obama is expected to sign, also orders the Federal Aviation Administration to develop regulations for the testing and licensing of commercial drones by 2015.
Privacy advocates say the measure will lead to widespread use of drones for electronic surveillance by police agencies across the country and eventually by private companies as well.
"There are serious policy questions on the horizon about privacy and surveillance, by both government agencies and commercial entities," said Steven Aftergood, who heads the Project on Government Secrecy at the Federation of American Scientists.
The Electronic Frontier Foundation also is "concerned about the implications for surveillance by government agencies," said attorney Jennifer Lynch.
The provision in the legislation is the fruit of "a huge push by lawmakers and the defense sector to expand the use of drones" in American airspace, she added.
According to some estimates, the commercial drone market in the United States could be worth hundreds of millions of dollars once the FAA clears their use.
The agency projects that 30,000 drones could be in the nation's skies by 2020.
The highest-profile use of drones by the United States has been in the CIA's armed Predator-drone program, which targets al Qaeda terrorist leaders. But the vast majority of U.S. drone missions, even in war zones, are flown for surveillance. Some drones are as small as model aircraft, while others have the wingspan of a full-size jet.
In Afghanistan, the U.S. use of drone surveillance has grown so rapidly that it has created a glut of video material to be analyzed.
The legislation would order the FAA, before the end of the year, to expedite the process through which it authorizes the use of drones by federal, state and local police and other agencies. The FAA currently issues certificates, which can cover multiple flights by more than one aircraft in a particular area, on a case-by-case basis.
The Department of Homeland Security is the only federal agency to discuss openly its use of drones in domestic airspace.
U.S. Customs and Border Protection, an agency within the department, operates nine drones, variants of the CIA's feared Predator. The aircraft, which are flown remotely by a team of 80 fully qualified pilots, are used principally for border and counternarcotics surveillance under four long-term FAA certificates.
Officials say they can be used on a short-term basis for a variety of other public-safety and emergency-management missions if a separate certificate is issued for that mission.
"It's not all about surveillance," Mr. Aftergood said.
Homeland Security has deployed drones to support disaster relief operations. Unmanned aircraft also could be useful for fighting fires or finding missing climbers or hikers, he added.
The FAA has issued hundreds of certificates to police and other government agencies, and a handful to research institutions to allow them to fly drones of various kinds over the United States for particular missions.
The agency said it issued 313 certificates in 2011 and 295 of them were still active at the end of the year, but the FAA refuses to disclose which agencies have the certificates and what their purposes are.
The Electronic Frontier Foundation is suing the FAA to obtain records of the certifications.
"We need a list so we can ask [each agency], 'What are your policies on drone use? How do you protect privacy? How do you ensure compliance with the Fourth Amendment?' " Ms. Lynch said.
"Currently, the only barrier to the routine use of drones for persistent surveillance are the procedural requirements imposed by the FAA for the issuance of certificates," said Amie Stepanovich, national security counsel for the Electronic Privacy Information Center, a research center in Washington.
The Department of Transportation, the parent agency of the FAA, has announced plans to streamline the certification process for government drone flights this year, she said.
"We are looking at our options" to oppose that, she added.
Section 332 of the new FAA legislation also orders the agency to develop a system for licensing commercial drone flights as part of the nation's air traffic control system by 2015.
The agency must establish six flight ranges across the country where drones can be test-flown to determine whether they are safe for travel in congested skies.
Representatives of the fast-growing unmanned aircraft systems industry say they worked hard to get the provisions into law.
"It sets deadlines for the integration of [the drones] into the national airspace," said Gretchen West, executive vice president of the Association for Unmanned Vehicle Systems International, an industry group.
She said drone technology is new to the FAA.
The legislation, which provides several deadlines for the FAA to report progress to Congress, "will move the [drones] issue up their list of priorities," Ms. West said.