Iran's president accused the United States and its allies on Wednesday of "hatching plots" to provoke ethnic tensions and destabilize Iran, a day after the government closed a state-run newspaper for publishing a cartoon that sparked riots by an ethnic minority.

"They (the U.S. and its allies) must know that they will not be able to provoke divisions and differences, through desperate attempts, among the dear Iranian nation," Mahmoud Ahmadinejad said in a speech broadcast live on state-run television.

CountryWatch: Iran

Iran closed the state-run Farsi language newspaper Iran and detained its chief editor and cartoonist on Tuesday for publishing a cartoon that showed a cockroach speaking Azeri and suggested that ethnic Azeris are stupid.

It was the first closure of a newspaper since Ahmadinejad came to office last year — and the heavy response, along with public apologies by Iranian officials, suggested the government is concerned the United States may try to stir up trouble among Iran's ethnic minorities.

"Today, they (the U.S. and its allies) are hatching plots. They want to provoke differences, divisions, disappointment ... to prevent the Iranian nation from achieving all of its rights," Ahmadinejad told thousands of people in Khorramshahr in southwestern Iran.

Other Iranian officials were also quick to stress the nation's unity in the standoff with Washington, which accuses Iran of seeking to develop nuclear weapons.

"It is clear that the evil hands of foreigners are making efforts to provoke tribal, ethnic, and religious differences under the present circumstances," State Public Prosecutor Ghorban Ali Dorri Najafabadi was quoted Tuesday as saying by state-run media.

Hundreds of Azeris marched on Monday in the northwestern city of Tabriz to protest the cartoon. Some broke windows at the governor's office, and police had to use tear gas to disperse the crowd, witnesses said. The violence was also reported by an independent news agency, ILNA.

Azeris, a Turkic ethnic group, are Iran's largest minority, making up about a quarter of Iran's 70 million people, dominated by ethnic Persians. Azeris speak a Turkic language shared by their brethren in neighboring Azerbaijan.

The cartoon, which ran May 12, showed people from different walks of life — including an athlete and a tradesman — trying to teach the cockroach and he always answers, in Azeri, "What do you mean?" There was no explanation why the protests broke out more than a week after the cartoon.

State television said the daily, one of the country's top three newspapers, had been closed "due to its publication of divisive and provocative materials." Tehran prosecutor Saeed Mortazavi said the paper's cartoonist and editor-in-chief had been taken to Evin prison.

Iran saw a wave of newspaper closures in past years amid the confrontation between reformers and hard-liners during the 1997-2005 presidency of reformist Mohammad Khatami. The hard-line judiciary shut more than 100 pro-reform newspapers and jailed dozens of editors and writers.

But the government of Ahmadinejad, an ultra-conservative, has taken no steps against the remaining two moderate newspapers that still publish — though they make few of the sharp criticisms of the cleric-led leadership that reformers did.

Instead, Tuesday's closure aimed to ensure national unity at a time when Iran takes seriously the possibility of U.S. military action over the nuclear issue.

A January article in The New Yorker magazine reported that U.S. military operatives were already working in Iran, making contacts with anti-government ethnic minorities and gathering intelligence. Defense Department officials said the article was filled with mistakes but did not deny its basic point. Iran denied that any U.S. agents had entered its territory.

Iran is a patchwork of many ethnic groups, but its numerous minorities have generally been quiet in past decades with little overt show of opposition to the government.

But there have been worrisome signs.

A series of bomb attacks have occurred in the past year in Khuzestan, the center of Iran's Arab minority, where Ahmadinejad spoke Wednesday. An Iranian Arab insurgent group claimed responsibility for a Jan. 24 blast that killed six people and wounded 46. The Iranian government blamed the bombings on Britain and United States, which denied any involvement.

In April, Iranian troops fired artillery and rockets at Iranian Kurdish guerrillas in Iraq amid reports they were crossing the border to infiltrate Iran's Kurdish minority.

On Tuesday, Interior Minister Mostaf Pourmohammadi warned that opportunists were working in Iran to inflame trouble.

"Some are misusing the situation. Others outside (Iran's) borders are trying to propagate protests through their satellite channels," he said in reference to U.S.-based Iranian opposition television channels.

"We should preserve our vigilance, especially the media ... and not allow, under the current very important situation the country is in now, others who pursue certain intentions to misuse the situation," he said.