The first shots were fired today by military forces imposing a no-fly zone over Libya as the international community swung into action against Muammar Gaddafi.
The show of strength against the Libyan leader began when a French jet attacked and destroyed a military vehicle belonging to his army.
Around 20 French Rafale and Mirage warplanes were sent to patrol the skies over the city of Benghazi after the rebel stronghold came under assault by forces loyal to Gaddafi in violation of United Nations resolutions.
RAF Typhoon and Tornado fighters were expected to join the mission shortly, supported by British reconnaissance and air-to-air refuelling planes, though it remained unclear exactly where they will be based and when they will be deployed.
The final decision to launch military action was taken at an emergency summit in Paris attended by world leaders, including Prime Minister David Cameron, who declared that "the time for action has come".
Along with European and North American allies, a number of Arab nations signed up to a communique pledging "all necessary action" to bring an end to the "grave and massive violations of humanitarian law" being committed by Gaddafi against his own people.
Countries including Canada, Denmark, Spain and Norway were sending planes, while Italy said it would permit the use of airbases such as Sigonella in Sicily and Aviano in the north to launch sorties.
Mr Cameron said: "Gaddafi has made this happen.
"He has lied to the international community, he has promised a ceasefire, he has broken that ceasefire, he continues to brutalise his people, and so the time for action has come.
"It needs to be urgent action. We must enforce the will of the UN. We cannot allow the slaughter of civilians to continue."
Mr Cameron returned to London to chair a meeting of the Government's Cobra emergencies committee in Downing Street.
He has promised MPs a debate and vote on military action in Libya in the House of Commons on Monday, but has made clear that he will not allow this to delay the use of force.
The launch of military operations follows Thursday's passage of United Nations (UN) Security Council resolution 1973, which authorised any measures short of foreign occupation to protect civilians in Libya.
The regime declared a ceasefire in response, but this appeared to have little effect on the ground, and the urgency of the situation was heightened this morning when Gaddafi's troops entered Benghazi, cradle of the month-long uprising against his 42-year rule.
A warplane filmed crashing in flames over the city appears to have been an opposition fighter accidentally shot down by its own side. There were also reports of Libyan civilians massing as "human shields" around sites which might be targeted in capital Tripoli.
Declaring the start of military operations, French President Nicolas Sarkozy said: "Our air force will oppose any aggression by Colonel Gaddafi against the population of Benghazi.
"As of now, our aircraft are preventing planes from attacking the town. As of now, other French aircraft are ready to intervene against tanks, armoured vehicles threatening unarmed civilians."
US Secretary of State Hillary Clinton later said that French warplanes were in the skies over Libya as the summit took place and made clear that operations will be stepped up shortly.
Mrs Clinton said that the US would use its "unique capabilities" to support air missions by European and Canadian allies and partners in the Arab world.
US officials were quoted as suggesting that this may involve missile attacks on Libya's air defences by American warships stationed in the Mediterranean.
It was hoped that today's developments will trigger further defections from the ranks of Gaddafi's forces, said Mrs Clinton. Left unchecked, the dictator could be expected to carry out "unspeakable atrocities", she warned.
Speaking during a visit to Brazil, US President Barack Obama said: "Our consensus was strong, and our resolve is clear. The people of Libya must be protected.
"In the absence of an immediate end to the violence against civilians, our coalition is prepared to act, and act with urgency."
Mr Cameron said that his thoughts were with "those who are going to risk their lives to help save the lives of others.
"Of course there are dangers, there are difficulties.
"There will always be unforeseen consequences of taking action, but it is better to take this action than to risk the consequences of inaction, which is the further slaughter of civilians by this dictator flouting the United Nations and its will."
Mr Sarkozy said that Gaddafi had "forfeited all legitimacy" but insisted that it was still not too late for the Libyan leader to avoid "the worst" by "complying immediately with and unreservedly with all the demands of the international community".
"The doors of diplomacy will open once again when the aggression stops," said the French president.
Gaddafi himself defiantly dismissed the UN resolution as "invalid" and warned that Britain would "regret" intervening in Libya.
In an open letter to Mr Cameron, Mr Sarkozy and UN Secretary General Ban Ki-moon, Gaddafi said: "Libya is not yours. Libya is for the Libyans. The Security Council resolution is invalid.
"You will regret it if you dare to intervene in our country."
In Tripoli, Libya's oil ministry urged Western firms which abandoned operations in the country at the outbreak of unrest last month to return, warning that contracts may otherwise be handed over to companies from countries like China and India, which are not backing the military action.
As allied military hardware assembled in the Mediterranean, Canadian CF18 planes stopped off to refuel at Prestwick Airport in Scotland en route for Italy. Canadian Prime Minister Stephen Harper said a sea blockade was also being prepared.
Spain is understood to have sent four F-18s and a Boeing 707 refuelling plane while Prime Minister Jose Luis Zapatero said he was deploying a submarine, a naval frigate and a surveillance plane to enforce an embargo on Libya.
Norway has said it is prepared to send six F-16 fighter jets.
German Chancellor Angela Merkel - whose ambassador abstained on the UN resolution on Thursday - said that the violence in Libya must be stopped, but ruled out any German involvement in military action.
The defence ministry in Paris later said French jets had "destroyed a number of tanks and armoured vehicles" belonging to Gaddafi's forces.
The International Committee of the Red Cross (ICRC) today called on all sides in Libya - including the multinational forces, Gaddafi's troops and the opposition - to abide by the principles of humanitarian law, particularly by distinguishing between civilians and fighters.
ICRC director-general Yves Daccord said: "Attacks that directly target the civilian population are strictly prohibited by international humanitarian law.
"That law also prohibits the use of human shields. Indiscriminate attacks are likewise strictly prohibited. The parties must take all precautions, including in their choice of means and methods of warfare, to avoid as far as possible harming civilians."
Combatants who surrender, are captured or are wounded must be treated humanely, he said.
And he added: "We urge the parties to the conflict to allow humanitarian organisations safe access to war-affected areas and to enable medical personnel and ambulances to reach the wounded."
The ICRC has been present in eastern Libya for more than three weeks, since shortly after violence broke out a month ago, supporting local medical teams and working closely with the Libyan Red Crescent.
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