The international community must seize the "massive opportunity" of the spread of democracy in the Arab Spring, David Cameron has told the UN.
In his first speech to the United Nations General Assembly, the UK PM urged the UN to be "united in action".
"To fail to act is to fail those who need our help," he told the Assembly in New York, saying the UN needed a "new way of working".
Mr Cameron has also had talks with President Barack Obama in the US.
In his address, the prime minister said nations must be prepared to act and not just sign resolutions. He said the UN had played a vital role in Libya but it was only effective when nation states were prepared to enforce its wishes.
Mr Cameron said: "My argument today is that Libya and the Arab Spring shows the UN needs a new way of working.
"Because the Arab Spring is a massive opportunity to spread peace, prosperity, democracy and vitally security, but only if we really seize it."
He went on: "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.
"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 people of the Arab world had made their aspirations for greater freedom, more accountable governments and an end to corruption clear.
"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.
He also said the UN had a responsibility "to stand up against regimes that persecute their people" and singled out Yemen and Syria as being in need of reform.
"Above all, on Syria, it is time for the Members of the Security Council to act.
"We must now adopt a credible resolution threatening tough sanctions. Of course we should always act with care when it comes to the internal affairs of a sovereign state. But we cannot allow this to be an excuse for indifference," he said.
Mr Cameron also told UN delegates that Palestinians had a right to a "viable state of their own" and that the international community should help them achieve this.
But Mr Cameron said that no resolution could provide the "political will" needed for lasting peace and called on both Israel and the Palestinians to return to the negotiating table for direct talks.
The BBC's Mark Urban said the Libyan intervention fitted well with Mr Cameron's campaign pledge to restore greater independence to Britain's foreign policy.
But he said many people at the UN listening to the speech would have regarded the UK action as "business as usual" by the West.
As he met Mr Obama at the Waldorf Astoria Hotel in New York earlier, Mr Cameron said that Libya was moving to a "good conclusion" after the ousting of Col Gaddafi.
He added that the UK and US were working closely together on Afghanistan and the Middle East peace process, and said the transatlantic relationship was as "strong as it's ever been".
The US president called it an "extraordinarily special relationship", adding: "I am very fortunate that over the last year or two David and I have been able to, I think, establish an excellent friendship as well."