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.

The barrage has effectively established a no-fly zone over Libya, halting an offensive by Gaddafi's forces against rebels in Benghazi, said Admiral Mike Mullen, the chairman of the US joint chiefs of staff.

"Operations [on Saturday] went very well," Mullen said on NBC. "He [Gaddafi] hasn't had aircraft or helicopters flying the last couple days. So effectively, that no-fly zone has been put in place."

Military officials are still assessing the damage after more than 110 Tomahawk cruise missiles were fired from US and British ships and submarines in the Mediterranean, targeting dozens of tanks, radar warning systems, surface-to-air missile launchers and communications centres.

Navy vice admiral William E Gortney, director of the Pentagon's joint staff, said officials were now reviewing satellite images to confirm the damage was extensive enough to allow the launch of air patrols to enforce a no-fly zone.

Tornado jets based in Norfolk took part in the air strikes, making it the RAF's longest-range bombing mission since the Falklands war.

On the road south of Benghazi, which had been attacked by Gaddafi's forces just hours earlier, a convoy of armoured vehicles was destroyed in an air strike before dawn on Sunday.

Strewn across the road lay the remains of five tanks and a rocket launcher. Half a dozen pickup trucks were consumed in a fireball and a flatbed truck was still in flames. Amid the wreckage lay dozens of bodies, some reduced to chunks of flesh.

State television said 48 people had died and 150 people were wounded in the attacks.

In a live address on Sunday, Gaddafi struck a defiant tone, claiming the air strikes were "simply a colonial crusader aggression that may ignite another large-scale crusader war".

Speaking from his bunker by telephone on state television, he said: "We are not afraid of your rockets or your air raids. We do not fear you. You were defeated in Somalia, in Lebanon, in Iraq. You were beaten by Bin Laden. We will defend our honour, our families, our homeland. We are ready for a long war."

The air strikes took place less than 48 hours after the United Nations 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, 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 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.

The 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 returning 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 said: "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."

About 20 French Mirage and Rafale fighter planes went into action over Benghazi,.

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."

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."

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 on Saturday afternoon, 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 on Saturday, a rebel plane was shot down over Benghazi, possibly in error by rebel forces themselves.