A British submarine fired Tomahawk cruise missiles on Libyan air defence targets as Prime Minister sent UK armed forces into action against dictator Muammar Gaddafi.
The launch of UK operations was announced by Mr Cameron outside 10 Downing Street after he chaired a meeting of the Government's Cobra emergency committee.
Hailing the military action as "necessary, legal and right", the Prime Minister said: "We should not stand aside while this dictator murders his own people".
The Ministry of Defence said that the Trafalgar-class sub had fired a number of land-attack missiles.
Meanwhile, the US confirmed that it had fired four missiles in what the Pentagon said was a "carefully co-ordinated" joint operation with the UK, France, Spain and Canada known as Odyssey Dawn.
More than 20 air defence systems on or near the coast had been struck, said a Pentagon spokesman.
Some 110 Tomahawk missiles were used in the attack on air-defence systems, which is the first stage of the allied operation, designed to suppress and degrade radar systems and surface-to-air missile sites in order to secure control of the airspace and reduce the threat to planes.
The UK submarine is understood to have been stationed in the Mediterranean for some time without being detected.
The Chief of Defence Staff's strategic communications officer Major General John Lorimer said: "I can confirm that British Armed Forces, as authorised by the United Nations Security Council Resolution 1973, have participated in a co-ordinated strike against Libyan air defence systems.
"The UK has launched guided Tomahawk Land Attack Missiles (TLAM) from a Trafalgar Class submarine as part of a co-ordinated coalition plan to enforce the resolution.
"This is the first stage. UK and partner forces remain engaged in ongoing operations as we seek to ensure that Colonel Gaddafi and his forces understand that the international community will not stand by and watch them kill civilians."
Mr Cameron said his thoughts were with British service personnel who were risking their lives to save others.
And he said: "I believe we should all be confident that what we are doing is in a just cause and in our nation's interest."
Earlier, around 20 French Mirage and Rafale fighter planes went into action over the rebel stronghold of Benghazi, which had been subjected to intense bombardment by Gaddafi loyalists despite a ceasefire announced yesterday.
The French fired the first shots of the operation, destroying tanks and armoured vehicles being used by forces loyal to Gaddafi.
It was not immediately clear whether RAF planes took part in the first wave of attacks. The Pentagon said that no US aircraft were deployed.
Mr Cameron yesterday told the House of Commons that RAF Typhoon and Tornado fast fighter jets would be involved in the mission - codenamed Operation Ellamy - alongside surveillance aircraft and air-to-air refuelling planes.
Military action was launched following an emergency summit in Paris, at which 19 nations - including a number of Arab states - agreed that Gaddafi had flouted the terms of Thursday's United Nations resolution demanding an immediate end to violence and authorising "any necessary measure" short of foreign occupation.
Mr Cameron hurried back from Paris to London to convene the Cobra meeting, bringing together senior ministers and military top brass to discuss the first military action of his time in office.
The Prime Minister said: "Tonight, British forces are in action over Libya. They are part of an international coalition that has come together to enforce the will of the United Nations and to support the Libyan people.
"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 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. It is legal, because we have the backing of the United Nations Security Council and also the Arab League and many others. And it is right because we believe we should not stand aside while this dictator murders his own people."
A Pentagon spokesman described this evening's missile strikes as "the first phase of a multi-phase operation".
The spokesman said: "Earlier this afternoon over 110 Tomahawk cruise missiles fired from both the US and British ships and submarines, struck more than 20 integrated air defence systems and other defence facilities ashore.
"These strikes were carefully co-ordinated with our coalition partners. The targets themselves were selected based a collective assessment that these sites either pose a direct threat to the coalition pilots or through use by the regime pose a direct threat to the people of Libya.
"Because it is night over there, it will be some time before we have a complete picture of the success of these strikes.
"I want to stress however that this is just the first phase of what will likely be a multi-phased, military operation designed to enforce the United Nations' resolution and deny the Libyan regime the ability to use force against its own people."
Most of the locations targeted by the air strike were either on or near the coast, making their destruction vital to the enforcement of the no fly zone, he said.
Unverified reports on Libyan TV claimed that air strikes by "the crusader enemy" have hit civilian areas in Tripoli and fuel storage tanks that supplied Misrata.
Thousands of Libyans were reported to have packed into Gaddafi's heavily fortified compound in the capital to form a human shield against possible attack by allied forces.
The launch of military operations followed 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.
Announcing 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."
Countries including Canada, Denmark, Spain and Norway announced they 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.
Declaring that "the time for action has come", 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."
US President Barack Obama, on a visit to Brazil, 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 Sarkozy said 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.
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."
In a radio address, US President Barack Obama said tonight: "Make no mistake. Today, we are part of a broad coalition. We are answering the calls of a threatened people."
A Libyan government spokesman said tonight that the attacks on his country were "barbaric" and have "no excuse".
Speaking at a news conference in Tripoli, he said: "I am very sorry and saddened that my country tonight is facing a barbaric and armed attack from Western countries."
He said a rocket attack had been launched on "several locations" including Tripoli and Misrata despite the Libyan authorities' announcement of a ceasefire, and claimed many civilians had been injured.Reuse content