Britain's Prime Minister David Cameron launched a personal attack on Mahmoud Ahmadinejad on Thursday minutes after the Iranian leader's anti-West speech caused a mass walkout.
Cameron made his debut address to the UN General Assembly just after Ahmadinejad's jibe at the United States and other western nations over the September 11, 2001 attacks and their role in wars and the financial crisis.
Britain's delegation joined a mass walkout by US and European diplomats.
Cameron said establishing democracy in Arab Spring countries would not rest on holding elections alone and went on to criticize Ahmadinejad.
"He didn't remind us that he runs a country where they may have elections of a sort but they also repress freedom of speech, do everything they can to avoid the accountability of a free media, violently prevent demonstrations and detain and torture those who argue for a better future," Cameron said.
"So we should never pretend that having elections is enough."
The British leader said pro-democracy uprisings in Libya, Tunisia and Egypt were "a challenge to Iran and Syria, to give their people the freedoms they deserve."
He praised the "coalition" of western and Arab nations that acted over Libya after the UN Security Council passed resolutions allowing for sanctions and military action to protect civilians.
"We must not now lose our nerve," said Cameron, who left for Canada shortly after his speech.
"We must have the confidence to speak out and act as necessary to support those who seek new freedoms."
He called for reform in Yemen, "and 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."
Britain, the United States and European powers have drawn up a resolution on Syria, but Russia and China are blocking sanctions.
Cameron also said the pro-democracy movements roiling North Africa and the Middle East posed "a challenge to the Israelis and Palestinians, to take the bold steps to come to the table and make lasting peace."
He did not say how Britain would vote in any resolution on Palestinian statehood, which the United States has vowed to veto. But Cameron said the Palestinians have a right to their own state and Israel security.
"There has been much speculation about what will happen here this week," he added.
"Let's be clear about one fact. No resolution can, on its own, substitute for the political will necessary to bring peace.
"Peace will only come when Palestinians and Israelis sit down and talk to each other, make compromises, build trust and agree. So our role is to support this, to defeat those who embrace violence, stop the growth of settlements and support Palestinians and Israelis alike to make peace."