The United States and Britain have launched cruise missiles against Libyan targets, a senior US military official has said.
More than 110 Tomahawk cruise missiles were fired from US and British ships and submarines, striking more than 20 air defence systems and facilities, a senior US military official said.
"This is just the first phase of what will likely be a multi-phase military operation designed to enforce the United Nations resolution and deny the Libyan regime the ability to use force against its own people," Vice Admiral Bill Gortney told a news conference at the Pentagon.
A coalition made up of the United States, Britain, France, Italy and Canada were involved in the UN-mandated military intervention, the US has said.
Some 25 coalition ships are stationed in the Mediterranean. Five US surveillance planes are also thought to be in the area.
The UN Security Council on Thursday voted for a no-fly zone over Libya and approved the use of "all necessary measures" - code for military action short of an invasion - to protect civilians against troops loyal to Libyan leader Colonel Muammar Gaddafi.
China and Russia were the most prominent voices in opposition to military action in Libya within the 15-member UN Security Council.
However, neither blocked the UN resolution authorising the operation, abstaining in the Security Council vote on the issue rather than using their veto power.
French planes fired the first shots in what is the biggest international military intervention in the Arab world since the 2003 invasion of Iraq, destroying tanks and armoured vehicles in the region of the rebels' eastern stronghold, Benghazi.
Canada is sending warplanes to the region, while Italy has offered the use of its military bases, the BBC reports. A naval blockade against Libya is also being put in place.
A US Pentagon official said the air strikes would mainly target air defences around Tripoli and Misrata in a joint operation called 'Odyssey Dawn'.
President Barack Obama said the US would not deploy any troops on the ground in Libya.
In an audio message, Colonel Gaddafi condemned the attacks as "barbaric, unjustified Crusaders' aggression," and said arms depots have been opened and "all the Libyan people are being armed".
Libyan state television said civilian areas of Tripoli and fuel storage tanks that supplied Misrata had been hit.
It said 48 people had been killed and 150 wounded by Sunday's air strikes.
Hundreds of Colonel Gaddafi's supporters have gathered at his Bab al-Aziziyah palace and the international airport to serve as human shields, state TV said.
Italian Prime Minister Silvio Berlusconi said Italy was providing bases for the airborne operation against Libya but could take part in raids if necessary, Reuters reported earlier.
An unidentified warplane was shot down over Benghazi. A rebel spokesman was quoted as saying the rebel forces had brought down their own jet by mistake.
The United Nations refugee agency says it is preparing to receive 200,000 people fleeing the fighting.
Protests against Colonel Gaddafi's 42-year rule began more than a month ago, and escalated into violent confrontations between opposition forces and government troops.