Britain walked out of a speech by President Mahmoud Ahmadinejad of Iran at the United Nations on Wednesday night in protest at anti-Semitic remarks.
By Alex Spillius in New York
South American delegations also marched from the grand hall at UN headquarters when the controversial leader denounced what he said was a global Jewish conspiracy, amid a long rant against capitalism and Western hypocrisy.
He denounced a "small minority dominating much of the world through a complicated network", and went on to call Iran a "glorious, democratic nation".
The United States, Canada and Israel decided earlier to boycott the speech before the annual UN General Assembly, after Mr Ahmadinejad repeated his denial of the Holocaust in a speech in Iran on Monday. He has also regularly called on Jews to leave Israel.
The discord over his appearance in New York came amid a heightened international effort to rein in Iran's suspected nuclear weapon plans.
The US, Britain, France, Germany, Russia and China – the so-called EU3 + 3 grouping that deals with Iran's nuclear threat, issued a statement on Wednesday night saying it expected a "serious response" from Iran at the October 1 meeting, when the US will join direct joint talks with Iranians.
Hillary Clinton, the US Secretary of State, said it was a "powerful statement".
"As a group we remain united in pressing Iran to comply with its international obligations," she said. "We want to see a serious effort by Iran to discuss the nuclear issue which we are putting on the table."
The Western powers in the group were encouraged by a suggestion from Dmitry Medvedev, the Russian president, that Moscow would be more prepared to accept impose sanctions.
Speaking after talks with President Barack Obama on the sidelines of the UN meeting, he said: "Sanctions rarely lead to productive results, but in some cases sanctions are inevitable."
Mr Medvedev also urged the international community to send the right signals and incentives to Tehran, which has always insisted its nuclear development is purely for peaceful purposes.
Russia, which has strong trade links with Russia and has supplied materials for its nuclear energy programme, has stiffly resisted sanctions so far.
"We need to help Iran to [make] the right decisions," he said.
Mr Obama meanwhile warned Iran that time was running out to open the doors of its nuclear programme to the rest of the world.
"Serious additional sanctions remain a possibility," the president said, calling on Tehran to "seize the opportunity" at the talks with the permanent five UN Security Council members plus Germany.
