BENGHAZI, Libya -- Tens of thousands of 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 in Tripoli 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 said. He added that many people were shot in the head, with seven people within 10 yards of him cut down in the first wave.

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

He said a series of financial sanctions he formalized Friday target Gadhafi's government while protecting the Libyan people's assets. The penalties name Gadhafi and several family members.

On 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 still could 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.

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."