* Verdict delayed until Sept. 4 as street protests simmer
* Military court convicted 20 revolt leaders last year
* Civilian retrial ordered in April
* U.S. wants detainees freed to stabilise ally against Iran
DUBAI, Aug 14 (Reuters) - Bahrain on Tuesday delayed until next month a ruling in the retrial of 20 men convicted of leading an uprising last year, lawyers said, a case under scrutiny from U.S. officials keen for acquittals to help restore calm in a regional ally against Iran.
Bahrain, where the U.S. Fifth Fleet is based, has been in turmoil since a protest movement dominated by majority Shi'ite Muslims erupted in February 2011 as part of the wave of popular revolts against repressive dynasties across the Arab world.
The 20 men - including seven being tried in absentia - are believed to be among hundreds who an international rights commission assessed in November had been tortured during a period of martial law imposed to help quell the uprising.
Bahrain's Sunni Muslim ruling family has faced calls from the United States for the release of all those jailed over their political views to defuse destabilising tensions and foster reconciliation and democratic reforms.
Riot police continued to clash almost daily with protesters in Shi'ite villages, while the interior ministry has banned requests for legal opposition rallies since June.
"The verdict was delayed to Sept. 4," said Mohammed Al-Jishi, a lawyer for some of the 13 men present in court for an expected reading of verdicts. The ruling kept the men in jail despite calls by protesters and rights groups for their release.
The presiding judge gave no reason for the postponement.
"It looks like the regime can't bring itself to take the hard decisions when it comes to reform and reconciliation," said Brian Dooley, director of the Human Rights Defenders Program at U.S.-based group Human Rights First.
The hearing was attended by a number of foreign diplomats, underlining how the outcome is expected to have a weighty impact on the evolution of the turmoil in the Gulf Arab island state.
After the postponement was announced, angry defendants chanted, "We sacrifice our soul and blood for you, Bahrain", visibly angering the judge, according to Jishi and other lawyers present in the courtroom.
CIVILIAN RETRIAL AFTER MILITARY CONVICTIONS
A military court sentenced the 20 men last year to jail terms of up to life for organising the 2011 uprising. A military appeals court upheld the sentences in September, but a civilian court ordered a retrial in April.
The main charges were "forming a terrorist group with intent to overthrow the system of government" as well as collaboration with a foreign state - an apparent reference to Shi'ite giant Iran across the Gulf from tiny Bahrain.
The defendants denied all such charges. Bahrain opposition leaders have generally called for full powers for the elected parliament to legislate and form governments.
Eight of the 20 men received life sentences, including rights activist Abdulhadi al-Khawaja and opposition leaders Hassan Mushaimaa and Abdulwahhab Hussein, who had called for turning the Gulf Arab monarchy into a republic.
Sunni opposition leader Ibrahim Sharif was sentenced to five years. Those tried in absentia include blogger Ali Abdulemam, who was given a 15-year sentence and is in hiding.
London-based Amnesty International said last week it hoped all Bahraini detainees would be released in Tuesday's session, saying they were "prisoners of conscience".
Analysts say the government - long dominated by the Sunni Al Khalifa family - is in a quandary over the case since the 20 men have become popular heroes whose release they fear could reinvigorate the protest movement and demands for reform.
The government has initiated some contacts with opposition parties on reforms but no formal public dialogue has transpired.
Bahrain is caught up in regional rivalry between Iran and Saudi Arabia. The presence of U.S. warships helps ensure a free flow of oil exports out of the Gulf, while Tehran has threatened a blockade if its stand-off with Western powers over its disputed nuclear programme deteriorates into confrontation.