Western allies unleashed a ferocious series of air and missile strikes against Muammar Gaddafi's military as they mounted Operation Odyssey Dawn, the biggest assault on an Arab regime since the invasion of Iraq in 2003.

A Pentagon spokesman said that more than 110 Tomahawk cruise missiles had been fired from US and British ships and submarines in the Mediterranean, striking more than 20 integrated air-defence systems and other military facilities on the mainland. A US national security official later said Gaddafi's air defences had been "severely disabled".

The spokesman said the attacks were just the start of what was likely to be a multi-phase military operation designed to enforce the will of the United Nations and "deny the Libyan regime the ability to use force against its own people".

The strikes took place less than 48 hours after the UN had authorised the use of "all necessary force" to protect civilians from Gaddafi's brutal attempts to regain control of his country. The patience of the international community finally ran out as Gaddafi's troops pounded the rebel stronghold of Benghazi, hours after the Libyan dictator had promised a full ceasefire and invited foreign officials to monitor it.

Powerful explosions heard to the west of the capital were thought to be an attack on a radar station – part of the country's air-defence system. A government spokesman said military and civilian locations had been hit.

Libyan television reported that the air forces of what it called the "crusader enemy" had hit a hospital on the outskirts of Tripoli. Targets in Zuwara and Misrata, Sirte and Benghazi were also hit, it said.

The Jana news agency reported that ambulances were rushing to take casualties to hospitals. But there were no air-raid sirens or overt signs of a country under bombardment. Traffic continued to flow as normal in the streets of Tripoli, with many vehicles flying the green flags of Libya and passengers screaming pro-Gaddafi slogans. A large crowd of Gaddafi supporters waving flags and portraits of the leader gathered outside his house in Tripoli, in an apparent move to discourage foreign air strikes.

US President Barack Obama said: "Make no mistake. Today we are part of a broad coalition. We are answering the calls of a threatened people."

After rushing back from a meeting of international leaders in Paris, where military action was agreed, to chair a meeting of the emergency Cobra committee, prime minister David Cameron declared: "Tonight British forces are in action over Libya. We have all seen the appalling brutality that Colonel Gaddafi has meted out against his own people. And far from introducing the ceasefire he spoke about, he has actually stepped up the attacks and the brutality that we can all see.

"So what we are doing is necessary, it is legal, and it is right. It is necessary because, with others, we should be trying to prevent him using his military against his own people."

Around 20 French Mirage and Rafale fighter planes went into action over Benghazi, which had been subjected to intense bombardment by Gaddafi loyalists despite the supposed ceasefire.

Later the BBC said Gaddafi had issued a statement saying the Mediterranean had been turned into a war zone and that military depots had been opened to allow his people to defend themselves and their country.

The International Committee of the Red Cross (ICRC) 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.

The ICRC's 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."

A defiant Gaddafi repeated in a national address that he would arm civilians to defend Libya from what he called "colonial, crusader" aggression by western forces. "It is now necessary to open the stores and arm all the masses with all types of weapons to defend the independence, unity and honour of Libya," he said in an audio message broadcast on state television hours after the strikes began.

Gaddafi said Libya would exercise its right to self-defence under article 51 of the UN charter, adding that the Mediterranean and north Africa were now a battleground.

"The interests of countries face danger from now on in the Mediterranean because of this aggressive and mad behaviour," he declared.

"Unfortunately, due to this [action], marine and air targets, whether military or civilian, will be exposed to real danger in the Mediterranean, since the area of the Mediterranean and north Africa has become a battleground because of this blatant military aggression."

He called on Arab, Islamic, African, Latin American and Asian countries to "stand by the heroic Libyan people to confront this aggression, which will only increase the Libyan people's strength, firmness and unity". But Qatar announced last night it will participate in military action against Gaddafi. Prime minister Sheikh Hamad bin Jassim al-Thani described the situation as "intolerable".

The French president, Nicolas Sarkozy, said French aircraft were already in action by late afternoon to stop what he described as Gaddafi's "murderous madness".

He 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."

The US secretary of state, Hillary Clinton, speaking in Paris, said the people of Libya had appealed for help and the international community had been united in its determination to respond: "We have every reason to fear that, left unchecked, Gaddafi will commit unspeakable atrocities. His campaign of violence must stop."

A US official said that America intended to limit its involvement to protecting allied air missions by taking out Libyan air defences with missile strikes launched from US ships stationed in the Mediterranean. Six Danish F-16 fighter jets landed at a US air base in Sicily, while Canadian CF-18 Hornets were also in the region.

The Gaddafi regime announced on Friday that it would lay down arms after the UN security council passed resolution 1973, authorising "all necessary measures" short of foreign occupation to protect the civilians of Libya. But as world leaders gathered in Paris, it was clear that forces loyal to the Libyan dictator had no such intention as they rushed to storm Benghazi – apparently in the belief that if they could embed themselves among the civilian population it would be more difficult for allied forces to oust them. Early yesterday, a rebel plane was shot down over Benghazi, apparently by Gaddafi's forces.

Fighting in Benghazi continued during even as French military jets began initial patrols over the city. It was not immediately clear whether a large explosion that rocked the edge of Benghazi after dusk was caused by Libyan forces on the ground or was the start of the western air assault.

"Libya is not yours. Libya is for the Libyans. The security council resolution is invalid," Gaddafi had written earlier in an open letter to Cameron, Sarkozy and UN secretary general Ban Ki-moon. "You will regret it if you dare to intervene in our country."

He was more conciliatory in a message to Obama, asking him: "If you found them [rebels] taking over American cities by the force of arms, tell me, what you would do?"

British sources said that Cameron had held discussions with Sarkozy before they joined Clinton for a session of the "leadership of the coalition of the willing". A session with all international leaders then took place in Paris.

Moussa Koussa, Gaddafi's foreign minister, insisted that Libya was abiding by the UN security council resolution passed last Thursday and that the ceasefire he announced on Friday was in place. Libya has blamed rebels it describes as "armed gangs linked to al-Qaida" for breaching it. "Our armed forces continue to retreat and hide, but the rebels keep shelling us and provoking us," it added.

But the ceasefire was widely seen as a tactic to try to buy time and fuel international divisions over intervention. That would have staved off the first foreign military action against Libya since the US bombing of 1986, after a terrorist attack on US personnel in a Berlin nightclub. The 1988 Lockerbie bombing was widely seen as Libya's retaliation for that.

Jana, the official Libyan news agency, reported that pro-Gaddafi volunteers were heading to strategic sites that might be targeted by UN-mandated attacks to act as "human shields". Al-Jamahirya TV showed protests at Tripoli international airport, Gaddafi's Bab al-Azizya barracks in the capital, and the airports in his home town of Sirte.A communiqué issued after the Paris talks described the actions of the Libyan regime as "intolerable". It said that 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."