The weapons have been seen by Human Rights Watch in two sites near Sirte, hometown of the late Gaddafi, containing surface-to-air missiles, tank and mortar rounds, munitions and thousands of guided and unguided aerial weapons.
"Surface-to-air missiles can take down civilian aircraft, and the explosive weapons can be converted easily into car bombs and IEDs (improvised explosive devices or bombs) that have killed thousands in Iraq and Afghanistan," Peter Bouckaert, the group's emergencies director, said in a statement.
He said Human Rights Watch had been warning the leaders of Libya's National Transitional Council (NTC) and its NATO supporters for months about stockpiles of unsecured weapons that had been regularly raided.
"Now that fighting has ended, one of the NTC's top priorities should be securing weapons facilities, and bringing the unchecked flow of arms in the country under control."
The NTC has publicly committed itself to securing stray stocks of Gaddafi's arsenal.
Gaddafi was captured and killed in unclear circumstances on Thursday when NTC forces captured Sirte, the last serious redoubt of his loyalists, and NTC leaders then declared the "liberation" of the vast North African state.
REGIONAL SECURITY THREAT
But the abandonment or disappearance of Gaddafi-era weapons has caused international concern that such firepower could erode regional security if it falls into the hands of Islamist militants or rebels active in North Africa.
Some also see a chance that remnants of Gaddafi loyalists or others unhappy with the faction-plagued NTC could use stray weaponry to wage guerrilla war, foiling effective government and a resumption of oil production in the OPEC-member country.
Human Rights Watch said U.S. experts were helping the NTC to locate missing surface-to-air missiles and along with Canada had offered money to help Libya destroy the weapons.
The group described seeing NTC fighters preparing to move weapons, including seven SA-24 surface-to-air missiles -- one of Russia's most advanced -- to their base in Misrata. The S-24s did not appear to have triggers, it said.
At one site, at least 28 SA-24 missiles seemed to be missing along with automatic rifles. At another, Human Rights Watch said it found a looted ammunition storage facility with crates of rocket-propelled grenades, anti-aircraft rounds and other munitions scattered around it in the surrounding desert.
"The evidence at these sites indicates that there is no time to waste," Bouckaert said.
In September, Bouckaert drew comparisons to Iraq, where storehouses left by fleeing troops loyal to Saddam Hussein after the 2003 American-led invasion were looted and used by militants to make suicide bombs.
The proliferation of surface-to-air missiles in Libya has worried Western security agencies who believe they could be used to target commercial airliners.