The missiles were reportedly fired from the Mediterranean Sea about 2 p.m. ET to clear the way for air patrols to ground Libya's air force.
Libyan state TV claimed 48 people had been killed in the attacks, but the report could not be independently verified.
The longtime Libyan leader vowed to defend his country from what he called "crusader aggression."
The move is the first direct U.S. involvement in the international mobilization effort to stop Libyan leader Moammar Gadhafi's attacks on rebel strongholds and impose a European-led no-fly zone over the North African country.
According to Pentagon officials, there were 11 U.S. ships stationed in the Mediterranean Sea, including three submarines and two destroyers capable of firing cruise missiles, as well as several amphibious ships and supply ships.
Navy Vice Adm. William E. Gortney, director of the Pentagon's Joint Staff, said the first strikes in what's being called Operation Odyssey Dawn would unfold in stages and target air defense installations around Tripoli, the capital, and a coastal area south of Benghazi, the rebel stronghold.
Gortney said the success of the mission was not immediately clear, adding that additional attacks would commence later. Gortney outlined two goals of the mission: prevent further attacks by Libyan forces on rebels and other civilians, and degrade the Libyan military's ability to contest a no-fly zone.
President Barack Obama, speaking during a visit to Brazil, said the United States and its allies had not sought this outcome but that Gadhafi had given the West no choice. In Obama's words: "We cannot stand idly by when a tyrant tells his people there will be no mercy."
Obama said he is aware of the risks of taking military action. He declared once again that the United States will not send ground forces into Libya.
Earlier Saturday French warplanes destroyed several Libyan military vehicles, including tanks, in eastern Libya today, French officials said.
French Defense Ministry spokesman Thierry Burkhard says the strike was reported around 1645 GMT Saturday. Burkhard says the target was confirmed as a military vehicle, but it was not clear what kind.
Obama said that the U.S. and its allies are prepared to act with urgency to end violence against Libyan civilians.
"Our consensus was strong, and our resolve is clear. The people of Libya must be protected, and in the absence of an immediate end to the violence against civilians our coalition is prepared to act, and to act with urgency," Obama said in Brazil, on the first day of a three-country Latin American tour.
Shortly after the president's remarks, U.S. Secretary of State Hillary Rodham Clinton spoke at a news conference in Paris after an international meeting on Libya. Clinton said the U.S. will bring "unique capabilities to bear" in Libya and said that the U.S. will support "all necessary measures" to put the U.N. Security Council resolution into place.
In a letter Gadafhi sent to the United Nations secretary general and French and British leaders, he warned: "You will regret it if you dare to intervene in our country."
Earlier Saturday, Libyan government troops forces stormed into the rebel capital of Benghazi, apparently ignoring a proclaimed cease-fire and potentially complicating any allied military action.
Also, a warplane was shot down over the outskirts of Benghazi, sending up a massive black cloud of smoke.
Before the plane went down, journalists heard what appeared to be airstrikes from it. Rebels cheered and celebrated at the crash, though the government denied a plane had gone down - or that any towns were shelled on Saturday.
The fighting galvanized the people of Benghazi, with young men collecting bottles to make gasoline bombs. Some residents dragged bed frames and metal scraps into the streets to make roadblocks.
At a news conference in Tripoli, a government spokesman read letters from Gadhafi to President Barack Obama and others involved in the international effort.
"Libya is not yours. Libya is for the Libyans. The Security Council resolution is invalid," he said in the letter to French President Nicolas Sarkozy, British Prime Minister David Cameron, and U.N. Secretary-General Ban Ki-Moon.
To Obama, the Libyan leader was slightly more conciliatory: "If you had found them taking over American cities with armed force, tell me what you would do."
In a joint statement to the Libyan leader on Friday, the United States, Britain and France - backed by unspecified Arab countries - called on Gadhafi to end his troops' advance toward Benghazi and pull them out of the cities of Misrata, Ajdabiya and Zawiya. It also called for the restoration of water, electricity and gas services in all areas. It said Libyans must be able to receive humanitarian aid or the "international community will make him suffer the consequences" with military action.
Parts of eastern Libya, where the once-confident rebels this week found their hold slipping, erupted into celebration at the passage of the U.N. resolution. But the timing and consequences of any international military action remained unclear.
Foreign Minister Moussa Koussa said that Libyan officials had informed the U.N. and the Security Council that the government was holding to the cease-fire and called for a team of foreign observers to verify that.
"The nation is respecting all the commitments put on it by the international community," he said, leaving the podium before answering any questions about Benghazi.
In Los Angeles, more than two dozen opponents of the Libyan dictator converged, waving American and Libyan flags and voicing their support for the missile strikes.
"I'm so happy," said demonstrator Fatima Shalluf of Torrance. "I hope it helps and I hope Gadhafi will finally leave so we can have a free and democratic Libya."
Experts say that the one thing that is certain is that Libya's political future must be determined by the Libyan people.
"Whether they're going to take the nationalist route, whether they're going to take a regionalist route, whether they split into even further fragments, we just can't force it," said Dr. James Gelvin, a professor of Middle Eastern history at the University of California, Los Angeles.
The Associated Press and ABC News contributed to this report.