Six human rights organizations and the government of Belgium will go to court here Wednesday in a long-shot effort to block the expected release of former Chilean president Augusto Pinochet. The groups decided to bring a legal action now because they feared they would not be able to reach a British judge fast enough to stop Pinochet from returning to Chile once a government order to free him is issued.

Home Secretary Jack Straw said two weeks ago that he is inclined to let Pinochet return to Chile without facing a trial on charges of torture and conspiracy brought by a magistrate in Spain. Acting on reports from a medical examination, Straw said the 84-year-old retired general is too sick to stand trial.

The human rights groups have challenged that conclusion and demanded a hearing into Pinochet's mental and physical condition. This week, they petitioned the High Court--despite its name, the basic trial court in the British system--for an order preventing Pinochet's departure until a hearing can be held. A hearing was scheduled for Wednesday morning.

Belgium joined the case because a Belgian prosecutor has indicted Pinochet on charges of human rights abuses during the 17 years he ruled Chile at the head of military junta. Belgium contends that if Pinochet goes home to Chile its prosecutors will never get him into a Belgian court.

The key problem for the anti-Pinochet side is that British extradition law gives the home secretary--a post akin to that of the U.S. attorney general--broad discretion in extradition cases. There is no statute, and apparently no case law, dictating how the home secretary should handle medical questions in an extradition case.

Accordingly, the plaintiffs are left arguing not on grounds of law but of fairness. "We're saying that Straw has not handled this fairly and the courts should not accept an arbitrary decision," said Kenneth Roth, executive director of New York-based Human Rights Watch.

The attack on Straw's fairness represents a clear reversal for the human rights community. Since Pinochet's arrest here 15 months ago, Straw has ruled against the Chilean at almost every turn, and human rights groups had praised the home secretary effusively. Pinochet has been under house arrest since October 1998 pending extradition to Spain for a criminal trial. After a convoluted legal odyssey, a British court ruled late last year that he can be extradited.

Although the human rights groups seem unlikely to prevail, the court case may delay Pinochet's departure for a few days at least. The Home Office said Straw will not issue a final decision until the High Court proceeding is finished.

The Belgian government said today it will also seek a ruling from the International Court of Justice in The Hague to keep Pinochet from leaving Britain.

CAPTION: Several anti-Pinochet protesters demonstrate outside the British Embassy in Brussels.