French planes are reportedly flying over Libya after world leaders decided the time for urgent action against Colonel Gaddafi has come.
They said he has flouted the UN's demand for an immediate end to violence against his people.
The French president Nicolas Sarkozy said "as of now" his country's air force is ready to attack Gaddafi's tanks and planes.
It follows a meeting of leaders from the UK, US, Europe and Arab countries in Paris.
Mr Sarkozy said Gaddafi's regime had forfeited all legitimacy but insisted that it was still not too late for the Libyan leader to avoid "the worst''.
He added: “Today we are intervening in Libya under the United Nations Security Council mandate, alongside our partners and our Arab partners.
“We are doing this in order to protect the civilian population from the murderous madness of a regime that by killing its own people has forfeited all legitimacy.
“We are intervening in order to enable the Libyan people to chose its own destiny.
“It must not be deprived of its rights by violence and terror.
“There is still time for Colonel Gaddafi 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.”
Gaddafi himself defiantly dismissed the UN resolution as “invalid” and warned that Britain would “regret it” if he intervened in Libya.
In an open letter to Mr Cameron, Mr Sarkozy and Mr Ban released in capital Tripoli today, 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.”
The Libyan leader was more conciliatory in a separate message to US President Barack Obama, asking him: “If you found them taking over American cities by the force of arms, tell me, what you would do?”
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 such as China and India, which did not back military action.
Mr Cameron yesterday said RAF Typhoons and Tornados, together with surveillance and air-to-air refuelling craft, would take part in the international effort to prevent Gaddafi committing a “bloodbath” against his own people.
The Ministry of Defence this morning declined to confirm whether any RAF planes had set off on their mission, codenamed Operation Ellamy, or where they would be based in the Mediterranean.
Canadian military planes landed at Prestwick Airport in Ayrshire today to refuel.
An airport spokesman said: “Prestwick is often used by military aircraft to refuel, that happens throughout the year.
“Military aircraft land to refuel and move on, so it‘s quite normal.”
Libya’s deputy foreign minister Khaled Kaim this morning told the BBC that the regime’s ceasefire was “real, credible and solid” and repeated an invitation for foreign observers to monitor it.
Mr Kaim insisted that Gaddafi’s air force had been grounded in response to Resolution 1973.
But he warned Libya would fight any attempt to intervene militarily in its territory. And he said foreign military intervention would suck countries from across the Arab world into the conflict.
“If there is an attack from outside or foreign intervention, you will see not only Libyans fighting foreign intervention, you will see people from Algeria, from Tunisia, from Egypt – and by the way Libyan tribes extend beyond Libya’s borders to Chad and Niger and Mali. All of them will be part of fighting against any foreign troops on Libyan ground,” said Mr Kaim.
A French official was reported to have said that the country's Mirage and Rafale fighter jets are flying over Benghazi and could strike Gaddafi's tanks today to prevent them from taking action there.
It had earlier been reported that French reconnaissance planes were flying over Libya.
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.
“We are united that the war must be ended,” Ms Merkel told reporters. “The resolution must be pushed through.
“We will not take part in the action in military terms. We will take on additional responsibilities in Afghanistan.”
A communique issued after the talks described the actions of the Libyan regime as ``intolerable''.
It said that, since February 15, the Libyan people had been peacefully expressing their rejection of their leaders and their aspirations for change, adding: “In the face of these legitimate requests coming from all over the country, the Libyan regime has carried out a growing brutal crackdown, using weapons of war against its own people and perpetrating against them grave and massive violations of humanitarian law.”
Despite the demands of the UN Security Council and condemnation from many governments across the world, the regime had stepped up its violence “in order to impose by force its will on that of its people”.
The communique demanded that Gaddafi and “those executing his orders” must immediately end acts of violence against civilians, withdraw from areas taken by force and “return to their compounds”.
It continued: “We reiterate that the Security Council took the view that Libyan regime’s actions may amount to crimes against humanity and that, to this end, it has referred the matter to the International Criminal Court.
“We are determined to take all necessary action, consistent with (the UN resolution) to ensure compliance with all its requirements.”
Assuring the Libyan people of support in building “their future and institutions within a democratic framework”, it praised the “courageous action” of the Libyan National Transition Council and “all the Libyans in positions of responsibility who have courageously disassociated themselves from the Libyan regime and given the NTC their support”.
The communique concluded: “Our commitment is for the long term: we will not let Colonel Gaddafi and his regime go on defying the will of the international community and scorning that of his people.
“We will continue our aid to the Libyans so that they can rebuild their country, fully respecting Libya’s sovereignty and territorial integrity.”
Unnamed US sources were reported as saying that American ships and aircraft are poised for action against Libya, but are not participating in the initial French air sorties.
One official said that the US intended to limit its involvement to protecting allied air missions by taking out Libyan air defences with missile strikes launched from US Navy ships stationed in the Mediterranean.
Six Danish F-16 fighter jets landed today at a US air base in Sicily while American F-18s and Canadian CF-18 Hornets were also in the region.
US President Barack Obama said the international community was resolved to protect the people of Libya.
Speaking during a visit to Brazil, the President 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.”
Secretary of State Hillary Clinton said: “The US will not deploy ground troops but there should be no mistaking our commitment to this effort.”
She told a press conference in Paris that the US would use its military capabilities to help its European and Canadian allies and Arab partners stop Gaddafi from attacking Libyan people.
Mrs Clinton said: “Colonel Gaddafi continues to defy the world and his attacks on civilians go on... As President Obama has said, we have every reason to fear that left unchecked, Gaddafi will commit unspeakable atrocities.
“We all recognise that further delay will only put more civilians at risk.”
She added: “Let me be clear about the position of the US. We will support an international coalition as it takes all necessary measures to enforce the terms of Resolution 1973.”
Shadow defence secretary Jim Murphy said: “This is a moment of truth for the international community to end the slaughter of innocents in Libya.
“The Government are right to take strong and urgent action against Colonel Gaddafi’s military capabilities. Gaddafi has no-one to blame but himself. Britain should stand in solidarity with the Libyan people.”
The Canadian military said the CF18 planes which landed at Prestwick were bound for Italy where they would be ready to assist the UN-backed no-fly zone over Libya.