David Cameron has urged the United Nations to be more ready to take action against oppressive regimes.
In a strong defence of interventionism, the Prime Minister said the Arab Spring uprisings and conflict in Libya demonstrated that the UN needed "a new way of working".
"To fail to act is to fail those who need our help," he told world leaders at the United Nations general assembly in New York.
Mr Cameron said the popular uprisings in north Africa and the Middle East had provided "a massive opportunity to spread peace, prosperity, democracy and vitally security" - but only if nations seized it.
"The UN has to show that we can be not just united in condemnation, but united in action, acting in a way that lives up to the UN's founding principles and meets the needs of the people," he said. "You can sign every human rights declaration in the world, but if you stand by and watch people being slaughtered in their own country, when you could act, then what are those signatures really worth?"
The Prime Minister said the international community needed to use a combination of military action or "hard power" and "soft power" like diplomacy and financial aid. He argued that the people of the Arab world had made clear their aspirations for greater freedom, more accountable governments and an end to corruption.
"As people in North Africa and the Middle East stand up and give voice to their hopes for more open and democratic societies, we have an opportunity - and I would say a responsibility - to help them," he said.
Mr Cameron said: "Here at the UN, we have a responsibility to stand up against regimes that persecute their people."
Asked by Channel 4 News whether he was blocking progress towards a Palestinian state, Mr Cameron said: "I am campaigning for Palestinian statehood. I want to help make it happen."
But he added: "We will only support measures and processes if they actually help to get these talks back on track. I'm a practical person, I don't believe in lofty declarations. I believe in rolling up sleeves to get things done."