WASHINGTON — The Obama administration shuttered the U.S. Embassy in Libya and froze all assets held in the United States by Libyan leader Moammar Khadafy, his government and four of his children, ending days of cautious condemnation by all but calling for the unpredictable leader’s immediate ouster.

Meanwhile, mercenaries and army forces put down an attempt by protesters Friday to break Khadafy’s hold on the capital, Tripoli, opening fire on crowds that had taken to the streets after prayers to mount their first major challenge to the government’s crackdown.

President Barack Obama said the new sanctions were justified by Libya’s “continued violation of human rights, brutalization of its people and outrageous threats” that have drawn condemnation from the world.

“By any measure, Moammar Khadafy’s government has violated international norms and common decency and must be held accountable,” Obama said in a statement announcing the penalties.

He said the sanctions were designed to target Khadafy’s government and protect the assets of Libya’s people from being looted by the regime.

The sharper U.S. tone and pledges of tough action came after American diplomatic personnel were evacuated from Tripoli aboard a chartered ferry and a chartered airplane, escorting them away from the violence to Malta and Turkey.

With U.S. diplomats and others out of harm’s way, the administration moved swiftly. Shortly after the chartered plane left Libyan airspace, White House spokesman Jay Carney said the U.S. had been constrained in moving against Khadafy and his loyalists because of concerns over the safety of Americans but was now ready to bring more pressure on the regime to halt its attacks on opponents.

The U.N. Security Council met to consider possible sanctions against Khadafy’s regime, including trade sanctions and an arms embargo.

Khadafy vows “hell”

The bloodshed in Tripoli heightened a standoff that has pitted Khadafy — who vowed Friday to turn Libya into “a hell” as he hunkered down in his stronghold — against a spreading rebel force and an alarmed international community.

A rebel officer who is coordinating an attack on Tripoli, Col. Tarek Saad Hussein, asserted in an interview that an armed, volunteer force of about 2,000 men — including army defectors — was to arrive in Tripoli on Friday night. There was no way to confirm his claim.

He was especially angered at the reports of security forces firing on protesters after prayers.

“They did not have weapons,” he said, speaking at an abandoned army base in the eastern city of Benghazi, which is under rebel control. “They shot people outside the mosque.”

Indeed, accounts of the bloodshed Friday indicated that Khadafy’s forces had deployed the same determined brutality as they had earlier in the week defending their leader, who has ruled for more than 40 years.

“They shoot people from the ambulances,” said one terrified resident, Omar, by telephone as he recalled an episode during the protests Friday when one protester was wounded. “We thought they’d take him to the hospital,” he said, but the militiamen “shot him dead and left with a squeal.”

Death toll unknown

Reports said several people were killed, but a precise toll might be impossible to know. Omar said that friends who were doctors at a hospital in Tripoli saw bodies being removed from the morgue to conceal the death toll. Local residents told him that the bodies were being taken to beaches and burned.

There was no way to confirm the account, and Omar did not want his full named used for fear of his life.

“We have no freedom here,” he said. “We want our freedom, too.”

Earlier Friday, Libyan state television showed Khadafy speaking from a parapet overlooking Green Square and addressing a crowd of supporters. He called on them to fight back and “defend the nation.”

“Retaliate against them, retaliate against them,” Khadafy said. Wearing a fur cap, he shook his fist, telling the crowd: “Dance, sing and prepare. Prepare to defend Libya, to defend the oil, dignity and independence.”

The New York Times contributed to this report.