Benghazi, Libya - Protesters demanding Moammar Gadhafi's ouster came under a hail of bullets Friday when pro-regime militiamen opened fire to stop the first significant anti-government marches in days in the Libyan capital. The Libyan leader, speaking from the ramparts of a historic Tripoli fort, told supporters to prepare to defend the nation.

Witnesses reported multiple deaths from gunmen on rooftops and in the streets shooting at crowds with automatic weapons and even an anti-aircraft gun.

"It was really like we are dogs," one man who was marching from Tripoli's eastern Tajoura district told The Associated Press. He added that many people were shot in the head, with seven people within 10 yards of him cut down in the first wave.

Also Friday evening, troops loyal to Gadhafi attacked a major air base east of Tripoli that had fallen into rebel hands.

A force of tanks attacked the Misrata Air Base, succeeding in retaking part of it in battles with residents and army units who had joined the anti-Gadhafi uprising, said a doctor and one resident wounded in the battle on the edge of opposition-held Misrata, Libya's third-largest city, about 120 miles from the capital.

The opposition captured two fighters, including a senior officer, and still held part of the large base, they said. Shooting could still be heard from the area after midnight. The doctor said 22 people were killed in two days of fighting at the air base and an adjacent civilian airport.

In Washington, President Barack Obama signed an executive order Friday freezing assets held by Gadhafi and four of his children in the United States. The Treasury Department said the sanctions against Gadhafi, three of his sons and a daughter also apply to the Libyan government.

Obama said the U.S. is imposing unilateral sanctions on Libya because continued violence there poses an "unusual and extraordinary threat" to America's national security and foreign policy.

A White House spokesman said it is clear that Gadhafi's legitimacy has been "reduced to zero" - the Obama administration's sharpest words yet. The U.S. also temporarily abandoned its embassy in Tripoli as a final flight carrying American citizens departed from the capital.

U.N. weighs sanctions

The U.N. Security Council met to consider possible sanctions against Gadhafi's regime, including trade sanctions and an arms embargo. Secretary-General Ban Ki-moon urged it take "concrete action" to protect civilians in Libya, saying "the violence must stop" and those responsible for "so brutally shedding blood" must be punished.

But Gadhafi vowed to fight on. In the evening, he appeared before a crowd of more than 1,000 supporters in Green Square and called on them to fight back and "defend the nation."

"Retaliate against them, retaliate against them," Gadhafi said, speaking by microphone from the ramparts of the Red Castle, a Crusader fort overlooking the square. 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.

"At the suitable time, we will open the arms depot so all Libyans and tribes become armed, so that Libya becomes red with fire."

The crowd waved pictures of the leader and green flags as he said, "I am in the middle of the people in the Green Square. ... This is the people that loves Moammar Gadhafi. If the people of Libya and the Arabs and Africans don't love Moammar Gadhafi, then Moammar Gadhafi does not deserve to live."

Gadhafi's son, Seif al-Islam, told foreign journalists invited by the government to Tripoli that there were no casualties in Tripoli and that the capital was "calm."

"Everything is peaceful," he said. "Peace is coming back to our country."

Seif said the regime wants negotiations with the opposition and said there were "two minor problems" in Misrata and Zawiya, another city near the capital held by the opposition. There, he said, "we are dealing with terrorist people." But he said he hoped to reach a peaceful settlement with them "and I think by tomorrow we will solve it."

Earlier Seif was asked in an interview with CNN-Turk about the options in the face of the unrest. "Plan A is to live and die in Libya, Plan B is to live and die in Libya, Plan C is to live and die in Libya," he replied.

The marches in the capital were the first major attempt by protesters to break a clampdown that pro-Gadhafi militiamen have imposed on Tripoli since the beginning of the week, when dozens were killed by gunmen roaming the street, shooting people on sight.

In the morning and night before, text messages were sent around urging protesters to stream out of mosques after noon prayers, saying, "Let us make this Friday the Friday of liberation," residents said. The residents and witnesses all spoke on condition of anonymity for fear of retaliation.

Heavy security

In response, militiamen set up heavy security around many mosques in the city, trying to prevent any opposition gatherings. Armed young men with green armbands to show their support for Gadhafi set up checkpoints on many streets, stopping cars and searching them. Tanks and checkpoints lined the road to Tripoli's airport, witnesses said.

After prayers, protesters flowed out of mosques, converging into marches from several neighborhoods, heading toward Green Square. But they were hit almost immediately by militiamen, a mix of Libyans and foreign mercenaries.

"We can't see where it is coming from," another protester from Tajoura district - several miles from Green Square - said of the gunfire. "They don't want to stop." He said a man next to him was shot in the neck.

In the nearby Souq al-Jomaa district, witnesses reported four killed as gunmen fired from rooftops. "There are all kind of bullets," said one man in the crowd, screaming in a telephone call to the AP, with the rattle of gunfire audible in the background. Another protester was reported killed in the Fashloum district. The reports could not be independently confirmed.

After nightfall, protesters dispersed and regime supporters prowled the streets, a resident said. As they have on past nights this week, many blockaded streets into their neighborhoods to prevent militiamen and strangers from entering.

Tripoli, home to about a third of Libya's population of 6 million, is the center of the eroding territory that Gadhafi still controls. The opposition holds a long sweep of about half of Libya's 1,000-mile Mediterranean coastline where most of the population lives.

Even in the Gadhafi-held pocket of northwestern Libya around Tripoli, several cities have also fallen to the rebellion. Militiamen and pro-Gadhafi troops were repelled Thursday when they launched attacks trying to take back opposition-held territory in Zawiya and Misrata in fighting that killed at least 30 people.

Support for Gadhafi continued to fray within a regime where he long commanded unquestioned loyalty.

Libya's delegation to the United Nations in Geneva announced Friday it was defecting to the opposition - and it was given a standing ovation at a gathering of the U.N. Human Rights Council. They join a string of Libyan ambassadors and diplomats around the world who abandoned the regime, as have the justice and interior ministers at home, and one of Gadhafi's cousins and closest aides, Ahmed Gadhaf al-Dam, who sought refuge in Egypt.

Libya's 11-member Arab League mission also announced its resignation in protest at the crackdown

The New York-based Human Rights Watch has put the death toll in Libya at nearly 300, according to a partial count from several days ago. Italy's Foreign Minister Franco Frattini said estimates of some 1,000 people killed were "credible."

The upheaval in the OPEC nation has taken most of Libya's oil production of 1.6 million barrels a day off the market. Oil prices hovered above $98 a barrel Friday in Asia, backing away from a spike to $103 the day before amid signs the crisis in Libya may have cut crude supplies less than previously estimated.

The opposition camp says it is in control of two of Libya's major oil ports - Breqa and Ras Lanouf - on the Gulf of Sidra. A resident of Ras Lanouf said Friday that the security force guarding that port had joined the rebellion and were helping guard it, along with residents of the area.

Several tens of thousands held a rally in support of the Tripoli protesters in the main square of Libya's second-largest city, Benghazi, where the revolt began.