The US and European nations pounded Moammar Gaddafi's forces and air defences with cruise missiles and airstrikes, launching the broadest international military effort since the Iraq war in support of an uprising that had seemed on the verge of defeat.
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" and warned the involvement of international forces will subject the Mediterranean and North African region to danger and put civilians at risk.
The US military said 112 Tomahawk cruise missiles were fired from American and British ships and submarines at more than 20 coastal targets to clear the way for air patrols to ground Libya's air force.
French fighter jets fired the first salvos, carrying out several strikes in the rebel-held east.
British military spokesman Maj Gen John Lorimer said British fighter jets also had been used to bombard the North African Nation.
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President Barack Obama said military action was not his first choice and reiterated that he would not send American ground troops to Libya.
"This is not an outcome the US or any of our partners sought," Obama said from Brazil, where he is starting a five-day visit to Latin America. "We cannot stand idly by when a tyrant tells his people there will be no mercy."
Thousands of regime supporters, meanwhile, packed into the sprawling Bab al-Aziziya military camp in Tripoli where Gadhafi lives to protect against attacks.
Anti-aircraft guns could be heard firing overnight in Tripoli. The strikes, which were aimed at enforcing a UN-mandated no-fly zone, were a sharp escalation in the international effort to stop Gadhafi after weeks of pleading by the rebels who have seen early gains reversed as the regime unleashed the full force of its superior air power and weaponry.
Libyan TV quoted the armed forces command as saying 48 people were killed and 150 wounded in the allied assault. It said most of the casualties were children but gave no more details.