David Cameron has told the UN General Assembly that countries must act to protect lives threatened by oppressive regimes.
The Prime Minister said the Arab Spring had shown the UN needed a new way of working.
Mr Cameron 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?"
He continued: "The UN has to show that we can be not just united in condemnation, but united in action."
The PM paid tribute to the "incredible courage" of the Libyans who had overthrown the Gaddafi regime.
He also warned it would be unwise to try to implement western values on countries who were part of the Arab Spring.
He said: "We should not be trying to impose western values or a single template on the region. Democracy is a process, not an event."
The PM criticised President Ahmadinejad of Iran whose earlier comments had provoked a walkout by western delegates.
Mr Cameron also called on Iran and Syria to "give their people the freedoms they deserve".