Northern Ireland authorities, battling to combat sectarian and political violence, have launched a hard-hitting television campaign to persuade people to inform on terrorists. A 60-second advertisement, being shown late in the evening because it features graphic scenes of violence, urges people to use a confidential police telephone line. "You know you can stop it--act now," the message says. The campaign begins after the first bloody weekend of violence in 1988 when three people--a young Roman Catholic man and two part-time British soldiers--were killed. Ninety-three people were killed in the troubled province in 1987.