Canadian authorities took a giant step forward this weekend in cracking down on the underground terrorist group whose actions the government cited more than three weeks ago in placing the country under military rule.

Police arrested Bernard Lortie, a 19-year-old student and a member of the Chernier cell of the Front de Liberation du Quebec, in an apartment near Montreal University last Friday.

Testifying before a coroner's court Saturday, Lortie confessed to the kidnapping of Quebec Labor Minister Pierre Laporte last Oct. 10. He also confirmed that three of the four prime suspects now being sought in connection with the case are members of the Chernier cell and that they too took part in Laporte's abduction.

At the same, Lortie denied any knowledge of who might have been responsible for Laporte's murder.

Prime Minister Pierre Elliot Trudeau imposed martial law on Canada last Oct. 16, following the kidnappings of Laporte and British Trade Commissioner James R. Cross by two separate cells of the FLQ. The law declared illegal membership in the secessionist group or support for its activities or goals.

Laporte was murdered less than two days later. Photograhs released by the FLQ's Liberation cell over the weekend indicate that Cross is still alive.

Lortie's capture and confession mark the first substantive government success in uncovering the membership of the FLQ. Following the invocation ofthe War Measures Act, police arrested only a handful of publicly known FLQ spokesmen and instead cracked down on hundreds of Quebecoises whose activities border on support for the group.

Most of the 400 persons arrested under the act have already been released, but police are still holding about 60 for questioning and arraignment. Three FLQ spokesmen have been charged with "seditious conspiracy," which carries a 14-year prison term and is a criminal offense unrelated to the specific sanctions of the emergency legislation.

Trudeau proposed last Monday the replacement of the War Measures Act with another law which slightly relaxes police powers and limits the former law's sweeping definition of who is a member of the FLQ. The proposal is expected to take effect sometime this week.