US and UK cruise missiles hit more than 20 Libyan air defence targets, Pentagon officials said
The UK, the US and France have begun attacking Libya as enforcement of the UN-mandated no-fly zone gets under way.
Pentagon officials say the US and the UK have fired more than 110 missiles.
Libyan leader Col Muammar Gaddafi has said he will open arms depots to the people and the interests of Mediterranean countries will be in danger from now on.
Western planes bombed targets in the capital, Tripoli, said the AFP news agency, quoting witnesses and state TV.
Earlier on Saturday, pro-Gaddafi forces attacked the rebel stronghold of Benghazi despite declaring a ceasefire a day earlier.
A French plane fired the first shots against Libyan government targets at 1645 GMT, destroying a number of military vehicles, according to a military spokesman.
UK Prime Minister David Cameron confirmed that British planes are in action over Libya.
US President Barack Obama, speaking during a visit to Brazil, said the US was taking "limited military action" as part of a "broad coalition".
"We cannot stand idly by when a tyrant tells his people there will be no mercy," he said.
He repeated that no US ground troops would take part.
Shortly after the bombing started, a Libyan official went on TV to denounce the "barbarian aggression".
Col Gaddafi himself made a brief speech on Libyan radio, calling on people to resist.
"Civilian and military targets in the air and sea will be liable to serious danger in the Mediterranean," he said.
"Arms depots are now open and the masses are being equipped with all sorts of weapons in defence of Libya's independence, unity and honour," the Libyan leader warned.
'Necessary'
A British submarine has fired a number of missiles at Libyan air defence targets, the Ministry of Defence said.
Mr Cameron said that launching military action against Libya was "necessary, legal and right".
Libyan state TV reported that what it called the "crusader enemy" had bombed civilian areas of Tripoli, as well as fuel storage tanks supplying the western city of Misrata.
Sources in Tripoli told BBC Arabic that the attacks on the city had so far targeted the eastern areas of Sawani, Airport Road, and Ghasheer. These are all areas believed to host military bases.
The air strikes on Misrata targeted a military airbase, the Reuters news agency reported, quoting two residents who denied the state TV reports that fuel stores were hit.
The cruise missiles were fired from one British submarine and a number of American destroyers and subs, said a Pentagon official.
The missiles hit more than 20 air defence sites along the Mediterranean coast, said Navy Vice Adm William E Gortney.
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In Libya, the civilian population, which is demanding nothing more than the right to choose their own destiny, is in mortal danger”
French planes also flew reconnaissance missions over "all Libyan territory", military sources in Paris said earlier.
In addition, Canada is sending warplanes to the region, while Italy has offered the use of its military bases. A naval blockade against Libya is also being put in place.
The international community was intervening to stop the "murderous madness" of Col Gaddafi, French President Nicolas Sarkozy said.
"In Libya, the civilian population, which is demanding nothing more than the right to choose their own destiny, is in mortal danger," he warned. "It is our duty to respond to their anguished appeal."
Shortly after the airstrikes began, Libyan state TV said a French plane had been shot down near Tripoli. However, French military officials said all their planes had returned safely.
Col Gaddafi has ruled Libya for more than 40 years. An uprising against him began last month after long-time leaders of neighbouring Tunisia and Egypt were toppled.
