The four were arrested last year during contentious iron ore contract negotiations.

Reporters outside the court said some journalists, mostly from Chinese state media, were allowed in to cover the opening session.

However, China has said the hearings on the industrial secrets charges will be held behind closed doors, raising questions from Australian officials over whether the men will get a fair trial.

Ahead of the trial Australia's prime minister, Kevin Rudd, warned China that the world would be watching the case "very closely".