The increasingly militant campaign of the world's leading voluntary euthanasia society to give incurably ill people assistance in committing suicide has run into strong legal opposition in Britain.

The leader of the London-based group -- Exit: The Society for the Right to Die with Dignity -- and one of its more than 10,000 members face criminal charges of illegally conspiring to aid and abet a number of suicides here.

The society also is fighting a lawsuit by another of its members, a physician, who is trying to stop it from distributing a booklet that contains detailed instructions on ways to commit suicide quickly, quietly, and painlessly.

These court actions follow a surge of international interest in recent years in voluntary euthanasia for the terminally ill and in the activities of the 45-year-old British society, which has spawned similar groups in 15 other countries, including the United States.

The central figure in the court battles here is Exit's general secretary, Nicholas Reed, 33, an intense academic with a flair for debate and publicity. He has transformed the group from a quiet, upper-class gentleman's philosophical society seeking changes in suicide laws into a broadly based, aggressive and controversial champion of the right of the incurably ill and disabled to end their suffering by suicide.

Under Reed's leadership, Exit has multiplied its paid membership five-fold, gained wide media exposure, held seminars and spent speakers to civic associations, campuses and radio and television programs throughout the country.

Exit has widely distributed voluminous literature arguing its cause and seeking new members, and contributions.

It also provides forms for witnessed declarations "to my family and my physician" that "if there is no reasonable prospect of my recovery from physical illness or impairment expected to cause me severe distress or to render me incapable of rational existence, I request that I be allowed to die and not be kept alive by artificial means and that I receive whatever quantity of drugs may be required to keep me free from pain or distress even if the moment of death is hastened."

Exit has unsuccessfully tried to persuade Parliament to enact legislation that would go further by allowing people to make declarations authorizing their doctors to "administer euthanasia" by killing them or helping them commit suicide if they are "suffering from an incurable and distressing disease or disability," as judged by at least two physicians.

In the meantime, Reed wants Exit to distribute to its members the self-help suicide handbook, which would eliminate the need of outside assistance for people able to take their own lives. Exit officials said many of its new members joined just to receive the booklet.

But the booklet and Exit's activism have stirred strong opposition here from church leaders, the British medical association, the police, some of Exit's own older members and establishment voices such as The Times newspaper. The Times warned in a recent editorial that distribution of the suicide booklet to people "when they feel low and vulnerable . . . can only result in unnecessary deaths."

It also may be illegal. Suicide is no longer a crime in Britain, and no one can be prosecuted for unsuccessfully attempting it. But under the 1961 law that made suicide legal here, it is still a crime in England and Wales for anyone, including a physician, to "aid, abet, counsel, or procure" another person's suicide.

The old leadership of Exit, which was replaced by Reed's supporters last month, opposed distribution of the suicide handbook for fear that it violated this law. The new leadership has pledged to distribute it to members, even if it means going to jail. And a legally independent offshoot of Exit in Scotland, where the 1961 law does not apply, has started supplying the booklet to members there.

But a special squad of detectives in London believes some at Exit already have violated the law here by advising and assisting people to commit suicide. Earlier this year, the police raided Exit's offices, carted away documents and took Reed in for questioning.

This week, Reed and an Exit member, Mark Lyons, were charged with conspiring to assist a number of people in commiting suicide, at least one of whom died.

In one case, according to the prosecutor, Lyons first drugged a woman into unconsciousness and then put a polyethylene bag over her head in an unsuccessful attempt to hasten her death but "suffered considerable disappointment since his victim proved rather more difficult than he had anticipated."

Lyon's faces a charge of murder in the death of one woman, five charges of aiding and abetting suicides and four of conspiring to aid and counsel suicide. Police searching Lyon's home discovered "suicide kits," detailed diaries of his activities, and documents connecting him to Exit.

Referring to Lyons, the prosecutor told the judge, "We find it difficult to attribute any attitude of good, but rather his conduct seems to have been motivated more by pleasure which he received in assisting the ending of life."

Reed is accused of conspiring with Lyons in several cases of aiding and abetting suicide, although details of his alleged participation were not provided in court.

Reed would not discuss the case with reporters.

But an official of Exit, Marsh Dickson, said Reed had telephone contact with Lyons, once telling him, "I just cannot help you.It's against the law."

Dickson attributed Reed's prosecution to "senior police officers who are opposed to what we are doing and are determined to put a stop to it. They are trying to haul Nicholas into this to discredit him and the organization."