WASHINGTON (AP) _ A public interest group filed a lawsuit in federal court today to unseal transcripts involved in Lt. Col. Oliver North's attempt to block the criminal investigation of the Iran-Contra affair and a contempt proceeding involving him.

The public should not be ''shut out of the administration of justice and development of the law in what is one of the most publicized and momentous matters in the criminal justice system today,'' said the Ralph Nader group, Public Citizen, in its action.

At North's request, almost all of the proceedings involving North have been conducted in secret both before the U.S. District Court and the U.S. Court of Appeals. A motion to unseal the judicial records was filed in the appeals court.

Another lawsuit in the Iran-Contra affair was filed Thursday in the District Court. The National Security Archive, a non-profit group that collects and disseminates government documents about national security issues, is seeking the internal records and back-up papers of the Tower Commission. The suit names the executive office of the president and White House Counsel Arthur B. Culvahouse Jr. as defendants.

Culvahouse did not immediately return a telephone request for comment.

Public Citizen, in its action, says it ''does not seek access to those records ... that reveal matters occurring before the grand jury or confidential details of the Independent Counsel's investigation.''

But, said Public Citizen, it wants tape recordings and transcripts of closed court hearings and arguments, court orders and opinion, all other papers connected with legal issues.

In a closed hearing on May 8, Judge Aubrey Robinson of the District Court held North in contempt for refusing to comply with a grand jury subpoena for a handwriting sample and also decided North's challenge to the investigation of Independent Counsel Lawrence Walsh was premature.

Public Citizen said it first learned of the case when the appeals court issued an order that indicated it would accept some unsealed briefs from parties in the case.

''The public and interested parties were given no indication of the nature of the case, the identities of the parties, how it came to the Court of Appeals, what other issues were under consideration by the court - or even why the court had sealed other portions of the case,'' the suit said.

It added that many of the legal issues bear on the public debate over how well Walsh is carrying out the investigation.

The National Security Archive, a division of the Fund for Peace, published a massive chronology of the Iran-Contra affair, which it said is widely used by journalists, Walsh's investigators and the staffs of the congressional committees investigating the matter.

Its action said Culvahouse, ''without legal authority'' took custody of records after the Tower Commission finished its investigation and went out of business. It said that the White House counsel replied that his office was not an ''agency'' subject to the Freedom of Information Act.