A French plane has fired the first shots in Libya as enforcement of the UN-mandated no-fly zone begins.
The UK prime minister later confirmed British planes were also in action, while US media reports said the US had fired its first Cruise missiles.
The action came hours after Western and Arab leaders met in Paris to agree how to enforce the UN resolution.
It allows "all necessary measures" to protect civilians from Colonel Muammar Gaddafi's forces.
'Stop the bombardment'
The French plane fired the first shot in Libya at 1645 GMT and destroyed its target, according to a military spokesman.
French planes also flew reconnaissance missions over "all Libyan territory", French military sources said earlier.
Around 20 French aircraft were involved in Saturday's operation, the Reuters news agency reports.
French jets "destroyed a number of tanks and armoured vehicles", a defence ministry official told Reuters, adding that he could not immediately confirm the number.
Other air forces and navies are expected to join the French.
The US would use its "unique capabilities" to reinforce the no-fly zone, said US Secretary of State Hillary Clinton, warning that further delays would put more civilians at risk. However, Mrs Clinton said again that the US would not deploy ground troops in Libya.
A naval blockade is also being put in place, said Canadian Prime Minister Stephen Harper. France is sending its Charles De Gaulle aircraft carrier to the Libyan coast, a military spokesman said.
In other developments:
- Italy has offered the use of seven of its military bases which already house US, Nato and Italian forces
- Canada says its fighter jets have now reached the region but will need two days to prepare for any missions
Earlier, pro-Gaddafi forces had attacked the rebel stronghold of Benghazi - although the Libyan government denied launching any assault.
The international community was intervening to stop the "murderous madness" of Col Gaddafi, Mr 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."
UN Secretary General Ban Ki-moon said Libya's claims to have implemented a ceasefire were "troubling", the AFP news agency reported.
The lack of confidence was so great that he did not trust what the Libyan leadership was saying, Mr Ban added.
Russia regretted the decision by Western powers to take military action, a foreign ministry spokesman said. Russia abstained from the UN vote on the Libya resolution, but did not use its veto.
The rebels' leader had earlier appealed to the international community to stop the bombardment by pro-Gaddafi forces.
A jet also appears to have been shot down over Benghazi. A rebel spokesman was quoted as saying the downed jet was a rebel plane.
Reports from Benghazi suggest hundreds of cars packed with people were fleeing eastwards as fighting spread.
The United Nations refugee agency says it is preparing to receive 200,000 people fleeing the fighting, amid reports of hundreds of cars full of people heading for the Egyptian border, while others are attempting to flee on foot.
The first families had arrived at the Egyptian border, extremely frightened and traumatised, saying some of their homes have been completely flattened said UNHCR spokeswoman Elizabeth Tan.
However, the BBC's Ben Brown, who is at the border, says so far there are a handful of families, in addition to the migrant workers who have been there since the crisis started.