Some of President Reagan's top advisers have operated a virtual parallel government outside the traditional Cabinet departments and agencies almost from the day Reagan took office, congressional investigators and administration officials have concluded. Investigators believe that the advisers' activities extended well beyond the secret arms sales to Iran and aid to the contras now under investigation. Lt. Col. Oliver North, for example, helped draw up a controversial plan to suspend the Constitution in the event of a national crisis, such as nuclear war, violent and widespread internal dissent or national opposition to a U.S. military invasion abroad. The advisers conducted their activities through secret contacts throughout the government with persons who acted at their direction but did not officially report to them. The activities of those contacts were coordinated by the National Security Council, the officials and investigators said. There appears to have been no formal directive for the advisers' activities, which knowledgeable sources described as a parallel government. In a secret assessment of the activities, the lead counsel for the Senate Iran-contra committee called it a secret government-within-a-government." The arrangement permitted Reagan administration officials to claim that they were not involved in controversial or illegal activities, the officials said. "It was the ultimate plausible deniability," said a well-briefed official who has served the Reagan administration since 1982 and who often collaborated on covert assistance to the Nicaraguan contras.