"Today I authorised the armed forces of the United States to begin a limited military action in Libya in support of an international effort to protect Libyan civilians. That action has now begun," US President Barack Obama announced in Brasilia, Brazil.
More than 110 Tomahawk missiles from US and British ships and submarines hit about 20 Libyan air and missile defense targets, Vice Admiral William Gortney, director of the joint staff, told reporters at Pentagon.
These missiles were directed at the Libyan western shores around Tripoli and Misratah. Potential targets included surface-to-air missiles, as well as early warning radar and communications sites.
"The goals of these initial operations are essentially twofold: first, to prevent further attacks by regime forces on Libyan citizens and opposition groups, especially around Benghazi, and second, to degrade the regime's capability to resist the no-fly zone we are implementing under that United Nations resolution," he said.
Noting that Operation Odyssey Dawn is under the command of Army Gen Carter F Ham, commander of US Africa Command, Gortney said most of the targets were on or near the coast and around the Libyan capital of Tripoli.
"I want to stress this is just the first phase of what will likely be a multiphase military operation designed to enforce the United Nations resolution," he said.
Obama slammed Gaddafi for not heeding to UNSC demands and attacking his own people. "Despite the (Gaddafi) hollow words of his government, he has ignored that opportunity. His attacks on his own people have continued. His forces have been on the move. And the danger faced by the people of Libya has grown."
Reiterating the US will not deploy any troops on the ground in Libya, the president said the US will contribute its unique capabilities at the front end of the mission to protect Libyan civilians, and enable the enforcement of a no-fly zone that will be led by its international partners.
The US is acting with a broad coalition that is committed to enforcing United Nations Security Council Resolution 1973, which calls for the protection of the Libyan people, Obama said.
"That coalition met in Paris to send a unified message, and it brings together many of our European and Arab partners," he added.
US military forces are on the leading edge of the coalition operation, taking out Libya's integrated air and missile defence system. "The ordnance is aimed at radars and anti-aircraft sites around the capital of Tripoli and other facilities along the Mediterranean coast," the Pentagon said in a statement.
The US will conduct a damage assessment of the sites, which include SA-5 missiles and communications facilities.
Earlier, French warplanes reportedly hit four tanks used by Gaddafi's forces on the outskirts of the opposition stronghold of Benghazi, on a day when opposition fighters in the city came under constant artillery and mortar fire.
However, Libyan state television reported civilian targets in Tripoli had been bombarded, as well as fuel stores in Misurata. The state news agency reported that there had been "civilian casualties as a result of this aggression".
The action by coalition forces came after their leaders approved military strikes against Gaddafi's forces.
In London, UK Prime Minister David Cameron said British forces were in action over Libya. "What we are doing is necessary, it is legal and it is right... I believe we should not stand aside while this dictator murders his own people."
French President Nicolas Sarkozy, earlier, noted his country's warplanes are already targeting Gaddafi's forces.
Meanwhile, in a brief statement on state television, Gaddafi said the air strikes marked the beginning of another "crusade", adding that the Mediterranean and North Africa were being turned into a "battleground".
He said that arms depots should be opened so that Libyans may defend themselves, Al Jazeera reported.While, Mohammad al-Zawi, the secretary-general of the Libyan parliament, said his country was facing a "barbaric" attack, and reiterated that Libyan forces had been observing a "ceasefire".