DUBAI, June 13 (Reuters) - A Bahraini military court on Monday postponed a hearing in the trial of 20 doctors, seized during crackdowns on anti-government demonstrations, after their lawyers complained they had been tortured in custody.

The doctors were among dozens of medical staff rounded up in the Sunni-led Gulf kingdom, which hosts the U.S. Navy's Fifth Fleet, following protests that erupted in February among the country's Shi'ite majority.

The detainees, who face charges ranging from stealing medicine to stockpiling weapons to taking over a hospital, entered pleas of not guilty on Monday.

But a second part of the hearing, where the defence could have cross-examined prosecution witnesses, was deferred to June 20 over the lawyers' challenges to the court's jurisdiction and requests that their clients undergo independent medical examination.

"They were forced to confess. I am 100 percent sure that my clients are being tortured," one lawyer, who asked not to be named, said after the session, which was observed by U.S. and other foreign diplomats.

Relatives of some of the defendants told Reuters they were being prevented from bringing clothing to detainees, whom they believed were being kept naked to humiliate them and produce confessions.

Bahraini officials deny allegations of systematic torture of detainees and say that any instances of abuse will be investigated and prosecuted.

Officials also contest opposition figures suggesting that hundreds of Bahrainis are undergoing trial in military court for their alleged role in the demonstrations.

The doctors' lawyers said they had been denied access to counsel during initial hearings that prosecutors attended.

Bahrain's prosecutor-general has previously said lawyers can meet with detainees after an initial hearing, devoted in part to determining whether defendants have counsel.

The protests, which were crushed in March, saw demands for an end to sectarian discrimination that Bahrain's Shi'ites say they face, as well as calls for a constitutional monarchy. A few Shi'ite groups demanded the complete abolition of the monarchy.

Bahraini authorities say the demonstrators had a sectarian agenda and were backed by Shi'ite power Iran. The activists deny this. (Editing by Dan Williams)