We couldn't stand back and let Qaddafi murder his people

The British prime minister argues that intervention in Libya was necessary, legal and morally right.

On Saturday, British forces went into action over Libya as part of an international operation working with the US and others at the request of Arab nations acting to enforce the will of the United Nations.

In line with UN Resolution 1973, there were two aims to these strikes. The first was to suppress the Libyan air defences and make possible the safe enforcement of the no-fly zone. The second was to protect civilians from attack by the Qaddafi regime. Good progress has been made. Coalition forces have largely neutralised Libyan air defences and, as a result, a no-fly zone has effectively been put in place over Libya. It is also clear that coalition forces have helped avert a bloody massacre in Benghazi - perhaps only just in time.

It is important to recall how we arrived at such a grave and dangerous undertaking.

On Friday evening, President Obama, President Sarkozy and I spelt out the conditions which Colonel Qaddafi had to meet, under the requirements of international law set out by UN Security Council Resolution 1973. First, we said that a ceasefire had to be implemented immediately and that all attacks against civilians must stop. Second, we said that Col Qaddafi had to stop his troops advancing on Benghazi. Third, we said that Col Qaddafi had to pull his forces back from Ajdabiya, Misurata and Zawiya, establish water, electricity and gas supplies to all areas, and allow humanitarian assistance to reach the people of Libya.

Col Qaddafi responded by declaring a ceasefire but straightaway it was clear that he was breaking that promise. He continued to push his tanks towards Benghazi as quickly as possible, and to escalate his actions against Misurata. On Saturday alone, there were reports of dozens of people killed in Benghazi, and dozens more in Misurata.

It was necessary, legal and right that he should be stopped - and that we should stop him. Necessary because, with others, we should be trying to prevent him using military violence against his own people. Legal because we have the backing of the United Nations Security Council. And right because I believe we should not stand aside while this dictator murders his own people. And the Arab League and many others agree.

In the summit in Paris on Saturday, the secretary general of the Arab League and representatives of Arab States, including Qatar, the United Arab Emirates, Iraq, Jordan and Morocco asserted their support for - and I quote - "all necessary action, including military, consistent with UNSCR 1973, to ensure compliance with all its requirements".

The message in Paris was loud and clear. The international community had heeded the call of the Arab nations. Together we assured the Libyan people of our "determination to be at their side to help them realise their aspirations and build their future and institutions within a democratic framework".

The next phase of the operation will continue with the focus on maintaining the no-fly zone and protecting the civilian population. At the time of writing, Col Qaddafi is still arranging his forces to inflict further attacks on the civilian population - notably in Misurata - and we are determined to stop him. The message to those in Col Qaddafi's forces is clear: we will not tolerate attacks on civilians and those who support such attacks will be brought to account for their crimes. Now is the time for those involved in the Qaddafi regime to desert him: to put down their arms, walk away from their tanks and stop obeying orders from someone who has brutalised his own people.

No military actions are without risk, but we will of course do everything we can to avoid civilian casualties. Indeed, on Sunday evening our RAF pilots aborted their mission when they determined that there were civilians in close proximity to the identified military targets. This is a clear example of the lengths we will go to in trying to minimise the risk of civilian casualties.

It is important that in supporting the implementation of the resolution, the international system plans now for stabilising the peace that will follow. This could include rapidly restoring damaged infrastructure, keeping important services such as health and education running, reforming the security sector, and ensuring an open and transparent political process to elections. All this will take time and will require an internationally led effort. Britain will play its part in this.

In terms of what happens politically and diplomatically, what is crucial is that the future of Libya is for the people of Libya to decide, aided by the international community. The Libyan opposition have made it clear that they do not want to see a division of the country, and neither do we. They have also expressed a clear and overwhelming wish for Col Qaddafi to go. And we agree with that too.

The UN resolution is limited in its scope. This military mission is not about regime change: it is about protecting civilians. But we will continue to implement a wide range of tough sanctions designed to put pressure on the regime. Of course, no one can be certain of what the future can hold, but as we stand here today the people of Libya have a much better chance of determining their destiny.

I am sensitive to the fact that those watching footage of the bombing on TV will be painfully reminded of operations in Iraq in 2003. Some may look for comparisons or see a hidden agenda. I would like to point out that the UN resolution - which we, with the Lebanese, the US and French helped draft - makes it quite clear there will be no foreign occupation of Libya. The resolution both authorises and sets the limits of our action. Specifically it excludes an occupation force of any form, on any part of Libyan territory.

But the differences with Iraq go deeper. It is not just that this time, the action has the full and unambiguous legal authority of the United Nations, nor is it just that this time it is backed by Arab countries and by a broad international coalition. The point is this: there are millions in the Arab world who want to know that the UN, the US, the UK, the French and the international community care about their suffering and their oppression. The Arab world has asked us to act with them to stop the slaughter. We must answer that call.

David Cameron is the prime minister of Britain