The deaths pushed the overall estimated death toll to 99 in five days of unprecedented protests against the 42-year reign of Moammar Gadhafi. Government forces also wiped out a protest encampment and clamped down on Internet service throughout the North African nation.
"The blood of our martyrs is still leaking from coffins over the shoulders of the mourners," one female protester, who is also a lawyer, said while standing in front of about 20 coffins lined up in front of the Northern Court building in Benghazi, Libya's second-largest city and the epicenter of the current unrest.
Before Saturday's violence, Human Rights Watch had estimated at least 84 people have been killed.
Hospitals ran low on medical supplies and were packed with bodies shot in the chest and head, said the medical official who spoke on condition of anonymity because of fears of reprisal.
"Many of the dead and the injured are relatives of doctors here," the official, who provided the figure of 15 dead, told The Associated Press in a telephone interview. "They are crying, and I keep telling them to please stand up and help us."
Information is tightly controlled in Libya, where journalists cannot work freely, and some of the accounts could not be independently confirmed. Other information comes from opposition activists in exile.
Gadhafi has been trying to bring his country out of isolation, announcing in 2003 that he was abandoning his program for weapons of mass destruction, renouncing terrorism and compensating victims of the 1986 La Belle disco bombing in Berlin and the 1988 bombing of a Pan Am airliner over Lockerbie, Scotland.
Those decisions opened the door for warmer relations with the West and the lifting of U.N. and U.S. sanctions, but Gadhafi continues to face allegations of human rights violations in the North African nation.
British Foreign Secretary William Hague called reports of the use of snipers and heavy weapons against demonstrators in Libya "clearly unacceptable and horrifying," and criticized restrictions on media access.
Before the Internet was shut down, videos posted on a Facebook page showed Libyan protesters smashing a stone representation of the "Green Book," which is Ghadhafi's manifesto, as well as destroying billboards of the Libyan leader. Video of torched Revolutionary Committees buildings also were posted.
Protesters say that defiance is growing with the increasing bloodshed and attempts by authorities to silence them by offering financial compensation to relatives of the dead.
"Gadhafi's men came to us and tried to bribe many of our colleagues," said the female protester, but she added that the opposition would not agree to any negotiations with the regime because of the bloodshed.
Her account could not be verified independently but was identical to those of several others contacted by the AP.
Hatred of Gadhafi's rule has grown in Benghazi in the past two decades. Anger has focused on the shooting deaths of about 1,200 inmates - most of them political prisoners - during prison riots in 1996.
Families of the dead since then have been holding small demonstrations calling for the prosecution of those responsible for the killings. But the current protests have been larger, apparently spurred by revolts that ousted the Tunisian and Egyptian leaders.
"There's no turning back," said Mohammed Abdullah, a Dubai-based member of the Libyan Salvation Front. "It is over for Gadhafi."
According to several accounts, police in Benghazi initially followed orders to act against the protest but later joined with them because they belong to the same tribe and saw the foreign mercenaries taking part in the killings.
A similar scenario took place in other eastern cities, including Beyda, which once housed Libya's parliament before Gadhafi's military coup in September 1969 toppled the monarchy.
Protests spread to outside the southern city of Zentan and west to Mesrata, the third-biggest city in Libya.
"Now people are tearing down the posters of Gadhafi. This never happened before," a protester from Mesrata said by phone who did not want to give his name because of fear of reprisal.
The capital of Tripoli, however, remained a stronghold of support for Gadhafi, with security forces swiftly curbing small protests erupting in the outskirts. Secret police were heavily deployed on the streets, as residents kept their opinions and emotions secret.
Residents reported receiving short messages on their mobile phones warning about taking any action against Gadhafi, national security and the oil industry, which are among "red lines" in Libya that must not be crossed.
A female protester said she tried to rally people in the streets Friday but ended up among 150 protesters detained by police at the end of the day. She was let go because she was the sole woman among them.
"It is very, very difficult for protesters to appear in the streets of Tripoli, except at night. People are under siege and those who dare to show up are arrested," she said.
State-run media show only footage of the flamboyantly dressed Gadhafi, which it called "the inspiring leader," waving to hundreds of cheering loyalists.
Libyan author Hisham Matar, whose novel "In the Country of Men" was shortlisted for the 2006 Man Booker Prize, said the regime wants to make "an example of Benghazi."
"The danger now is that because of the extraordinary impunity with which the Gadhafi regime and security apparatus are able to act, we might see the death toll rise even higher," said Matar, whose father, a political dissident, was kidnapped in Egypt in 1990 and never seen again.