"Everybody should be alert in the region. All the Arab nations in the region -- Iraq, Lebanon and Palestine and other Muslim nations should be alert," he told thousands of worshippers gathered in Tehran for Eid-ul-Fitr prayers.
"They should be careful not to make any move that allows a new betrayal from the plans of the Zionists and the United States to be realised."
He warned Palestinians in particular to keep their unity amid deadly factional bickering between the mainstream Fatah faction and Hamas.
"The Palestinian brothers from the different groups and wings must know that their unity today is the most important factor in their victory," he said.
"They should not be ignorant about the enemies' desires to divide them and the conspiracies the enemies have thrown for them."
Khamenei said that following Hizbullah's "victory" in its latest conflict with Israel, the people of Lebanon should "keep their unity with strong calls for resistance."
"The Iraqi people should do everything to avoid sectarian conflict," he added, in reference to the deadly bloodletting between Sunnis and Shiites that is claiming dozens of lives daily.
"This (alertness) is the solution and the cure for the problems in our region," he added.

Copyright AFP (Agence France-Presse), 2006