BELGRADE, Yugoslavia (AP) _ Thousands of people in Slovenia, the nation's most liberal republic, have demanded the release of two journalists arrested after reporting the armed forces planned a coup in Slovenia.

Yugoslav President Raif Dizdarevic said an army tribunal's probe of Janez Jansa and David Tasic will be completed soon, a newspaper in Ljubljana, the capital of the western republic, reported Saturday.

The Delo newspaper said Dizdarevic assured Slovenian President Janez Stanovnik during a meeting in Belgrade on Thursday that ''the public would be informed fully as soon as the procedure is over.''

An army sergeant-major also was arrested, apparently on suspicion of providing Jansa with a secret document whose contents were not disclosed.

More than 7,000 people in Slovenia have signed a petition for the release of the three men and the arrests have led to a wave of protests in the republic.

More than 200 Communist Party cells, companies and trade union organizations are reported to have donated money into a special defense fund.

The two well-known reporters were arrested in Ljubljana on suspicion of revealing military secrets. Both journalists have written numerous articles criticizing the military. Jansa was taken into custody May 31 and Tasic was arrested five days later.

The coup reports were denied by the Yugoslav government, but copies of documents purported to be the minutes of a military advisory body's session and alleging that such a plan existed, have been widely circulated in Slovenia.

All three men were being held in a military prison in the Slovenian capital while the investigation is handled by the army's judicial branch.

Except for two brief communiques by the army prosecutor, the military has not revealed any details about the charges.

Jansa and Tasic were denied civilian defense attorneys, a highly unusual procedure in Yugoslavia and were held incommunicado until Thursday when family members were allowed to visit them.

Slovenia has long been more liberal than the rest of Yugoslavia and an irritant to Yugoslavia's communist establishment. Mladina, a youth weekly in the republic, has often criticized the army and advocated such liberal causes as conscientious objectors, homosexuals and the environment.