* About 30,000 Libyans protested after Benghazi attack
* Protests gives government opportunity to act on security
* Little progress in transferring detainees to government
By Michelle Nichols
UNITED NATIONS, Nov 8 (Reuters) - The fate of detainees held since the end of Libya's civil war is of "considerable gravity," the United Nations said on Thursday, and the government is facing growing public pressure to rebuild and reform the country's security authorities.
U.N. special envoy for Libya, Tarek Mitri, told the U.N. Security Council that after an attack on the U.S. consulate in Benghazi on Sept. 11 that killed the U.S. ambassador, about 30,000 Libyans took to the streets to demonstrate on Sept. 21.
Mitri said the demonstration showed "public pressure is mounting on the government to act decisively and quickly to build and reform the state security sector institutions."
"While this large outpouring of public support underscores the urgency of the issue, it provides the news government the opportunity to move swiftly and decisively in advancing security sector reform," he said.
U.S. President Barack Obama and other U.S. officials have acknowledged that the attack was a "terrorist" act by militants with suspected links to al Qaeda affiliates or sympathizers.
Muammar Gaddafi's 42-year rule collapsed when his forces fled Tripoli in August 2011, and the last of the fighting in Libya's nine-month civil war ended in October 2011 when he was captured and killed by rebels.
Thousands of detainees, many of whom are sub-Saharan Africans suspected of fighting for Gaddafi's government, were still being held in detention centers across the country, some operated by the government and some by revolutionary brigades.
The U.N. human rights agency and aid groups have accused the brigades of torturing detainees.
"The fate of those detained following the cessation of fighting is of considerable gravity, particularly given Libya's unresolved detention-related issues over the past year," Mitri said. "Little progress has been made in transferring detention facilities from brigrades to the Ministry of Justice.
"While the president and the prime minister provided assurances that all detainees would be placed under the authority of the judiciary, I stressed the need to institute measures to properly account for them and ensure their human treatment and afforded fair trial standards," he said.
Accusations of the mistreatment and disappearances of suspected Gaddafi loyalists have proved awkward for the Western powers that backed the anti-Gaddafi rebellion and installed interim leaders before Libya held a largely peaceful election in early July. That was its first national and free vote in 60 years.
Mitri said the "problems faced (by Libya) should not be underestimated but are not insurmountable."