Mr Cameron joined other world leaders - including representatives of several Arab states - at an emergency summit in Paris which agreed to deploy military aircraft to stop the assault on Benghazi.
Speaking after the summit, the PM said:
What is absolutely clear is that Qadhafi has broken his word, broken the ceasefire and continues to slaughter his own civilians.
This has to stop. We have to make it stop. We have to make him face the consequences. So I think it is vitally important that action takes place and action takes place urgently.
Obviously at this time our thoughts should be with those who are going to risk their lives to help save the lives of others
I think it is vitally important that, with the United Nations behind us, with the clear legality of this action, and with local countries supporting us as well, it is right to act.
Participants in the summit agreed “to use all necessary means” to enforce Thursday’s UN Security Council resolution on Libya.
Speaking later on outside downing Street, Mr Cameron stressed that what the coalition was doing was “necessary, legal and 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 of 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.
Read more: Joint statement on Libya from the UK, US and France with the support of Arab States
Read more: Communique: Paris summit for the support of the Libyan people