* Iran mass trials seen as sign hardliners feel insecure
* Khamenei urged by Revolutionary Guard to jail Mousavi
* Internal struggle dims chance for response to Obama offer
BEIRUT, Aug 9 (Reuters) - Iran's latest mass trial is meant to deter dissent but may also reflect insecurity among hardliners jolted by street protests and political splits after a disputed June election.
The crackdown shows no willingness to compromise on the part of re-elected President Mahmoud Ahmadinejad. He must now pick a cabinet and get it approved by a parliament with a conservative majority containing many who mistrust the firebrand leader.
Many conservatives were angered by the death in custody of the son of an aide to presidential candidate and former Revolutionary Guard commander Mohsen Rezaie, which may help explain why Iran's police chief announced on Sunday that the head of one notorious detention centre had been jailed.
But in other signals that the gloves have come off, a senior Guard commander demanded that Ahmadinejad's election rivals Mirhossein Mousavi and Mehdi Karoubi face trial, and a military commander urged greater control over foreign media.
"The sham trials are actually to shore up support among (Ahmadinejad) supporters who have begun having doubts," said Muhammad Sahimi, a professor at the University of Southern California.
An Iranian court charged a Frenchwoman, two Iranian staffers at the British and French embassies and dozens of others on Saturday with spying and plotting to overthrow clerical rule, in the second mass trial to open within a week.
Mousavi and Karoubi say the election was stolen to keep Ahmadinejad in power and have denounced the trials. They and influential backers like former presidents Mohammad Khatami and Akbar Hashemi Rafsanjani have solid revolutionary credentials.
"If Mousavi, Karoubi and Khatami are main suspects behind the 'soft revolution' in Iran, which they are, we expect the judiciary ... to go after them, arrest them, put them on trial and punish them," said Yadollah Javani, head of the Revolution Guard's political unit, the official IRNA news agency reported.
It is not clear whether Supreme Leader Ayatollah Ali Khamenei will yield to such demands to lock up mainstream opposition leaders in the world's fifth biggest oil exporter.
The influence of the Revolutionary Guard, a military force with extensive business interests, appears to have grown since Ahmadinejad, a former officer in the corps, took power in 2005.
But Khamenei, whose status as lofty arbiter has been eroded by his pro-Ahmadinejad partisanship, knows many senior clerics and politicians are alarmed at where the Islamic Republic is heading and are ready to protect their own interests.
Such divisions within the elite mean Mousavi may still be able to challenge the legitimacy of Ahmadinejad's government, said Mehrzad Boroujerdi, at New York's Syracuse University.
"The fact that the government has not arrested Mousavi and Karoubi suggests that they are worried about the repercussions."
"SIGN OF WEAKNESS"
U.S. Secretary of State Hillary Clinton called the first "show trial" a sign of weakness. "It demonstrates I think better than any of us could ever say that this Iranian leadership is afraid of their own people, and afraid of the truth and the facts coming out," she told CNN on Thursday.
Accusations that Western powers incited the post-election unrest further cloud prospects of Iran accepting U.S. President Barack Obama's offer of direct talks on nuclear work the West suspects is for making bombs, and not only to fuel power plants.
"The Iranian government is just not the same as it was before the election," said Gary Sick, an Iran scholar and former U.S. National Security Council official. "They are engaged in an existential crisis, a crisis of survival."
At least 26 people were killed and hundreds detained during protests involving hundreds of thousands of Iranians after the June 12 vote, which the authorities deny was fraudulent.
Security forces have reasserted control, but the violence deployed and the harsh treatment of detainees, several of whom died in custody, widened rifts in Iran's ruling establishment.
In an attempt to calm widespread anger, police chief Esmail Ahmadi-Moghaddam said on Sunday the head of the Kahrizak detention centre and three policemen there had been jailed. He acknowledged that some detainees had been tortured or beaten.
Khamenei had already ordered the Kahrizak centre closed. Sick said the turmoil had pushed decisions on nuclear policy and relations with the United States, which lie ultimately with Khamenei, lower down the leadership's priority list.
"What they have on their minds right now is very personal. It's a matter of the future of the movement and of particular individuals trying to hold on to power at all costs," he said.
For now, the hastily staged trials of reformist politicians, journalists, lawyers, academics and others, some of whom have made awkward confessions after weeks in jail with no access to lawyers, send a chilling message to Iran's diverse opposition.
Many of the accused face charges of espionage or acting against national security, both punishable by death.
"Iran could turn into a sort of authoritarian state in which all legitimacy is replaced by repression," Sick suggested.
"If so, it could go on for a very long time. It would be ugly and it would be difficult to deal with Iran during that time, but we would probably have to, whether we like it or not." (Editing by Robert Woodward)
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