Anti-aircraft fire erupted in the Libyan capital tonight with volleys of tracer fire arching into the air, marking the start of a second night of allied strikes aimed at enforcing a no-fly zone.
There was no immediate word on the targets in the latest round of strikes in the assault dubbed Operation Odyssey Dawn.
The heavy chatter of anti-aircraft defences in Tripoli began soon after nightfall.
The US military said the first air assault by the US and its allies last night - including airstrikes by long-range bombers and a shower of Tomahawk cruise missiles - was successful, though it did not fully eliminate the threat from Libyan air defences.
State television said 48 people had died in the first phase of the strikes launched to uphold the UN-mandated no-fly zone in support of rebels who have seen early gains reversed by Colonel Muammar Gaddafi regime's superior air power and weaponry.
In Benghazi, the rebel capital and first city to fall to the uprising, people said the international action had been launched just in time. Libyan government tanks and troops had reached the edges of the city yesterday.
But in a phone call to state television, Gaddafi said he would not let up on Benghazi. He said the government had opened up weapons depots to all Libyans, who were now armed with "automatic weapons, mortars and bombs".
Gaddafi said the international action against his forces was unjustified, calling it "simply a colonial crusader aggression that may ignite another large-scale crusader war".
US and British ships and submarines launched the first phase of the missile assault on Libyan air defences, firing 112 Tomahawk cruise missiles at more than 20 coastal targets to clear the way for air patrols to ground the country's air force.
French fighter jets fired the first salvos, carrying out several strikes in the rebel-held east, while British fighter jets also took part.
US President Barack Obama said military action was not his first choice and reiterated that he would not send American ground troops.
"This is not an outcome the US or any of our partners sought," President Obama said. "We cannot stand idly by when a tyrant tells his people there will be no mercy."
Thousands of regime supporters packed into the sprawling Bab al-Aziziya military camp in the capital Tripoli, where Gaddafi lives, to protect against attacks.
The International Committee of the Red Cross said it was "deeply concerned" about civilian casualties and called on all sides to work to distinguish between civilians and fighters and allow safe access for humanitarian organisations.
The allied assault followed an emergency summit in Paris during which the 22 leaders and top officials agreed to do everything necessary to make Gaddafi respect a UN Security Council resolution calling for the no-fly zone and demanding a ceasefire, said French President Nicolas Sarkozy.