NEW YORK -- Iranian President Mahmoud Ahmadinejad will encounter a turning point in his regime's relationship with the Iranian diaspora when he arrives here next week for the United Nations General Assembly.

Thousands of Iranians in exile are traveling to New York from cities across the U.S. and Canada to take part in demonstrations from Sept. 22-24 against the Iranian government and to protest its alleged human-rights abuses following disputed presidential elections in June.

For...