Nicaraguan Ambassador Carlos Tunnermann reluctantly yielded Friday to a Reagan administration demand that he leave the country, departing five minutes ahead of the deadline imposed by the State Department.

But he took a parting shot at the administration, insisting that the U.S. action threatened the independence of the Organization of American States and saying his leftist government would seek recourse through OAS legal channels.Speaking at a crowded embassy news conference shortly before leaving for the airport, Tunnermann said defiance of the expulsion order could give the Reagan administration an excuse "to continue its aggression against our people."

Tunnermann and seven embassy colleagues were notified of the expulsion on Tuesday and were given until 5:30 p.m. EDT Friday to leave the country. They flew to Mexico City via Houston on a 5:25 p.m. flight and planned to go to Managua on Saturday.

About a dozen police officers were stationed at the Continental Airlines plane's gate at Washington National Airport. Two police vehicles followed the jet out onto the taxiway, then pulled out to the head of the runway and waited for it to take off.

Tunnermann's wife, Rosa, who accompanied the envoy to the news conference, said she and seven of the couple's children plan to remain in the Washington area.

The State Department acted after eight American diplomats, including Ambassador Richard Melton, were ordered home by the Nicaraguan government on Monday for alleged interference in the country's internal affairs.

The administration described the U.S. response as retaliatory and said that in the case of Tunnermann and Nicaragua's military attache, Major Pedro Sampson, the expulsion was also based on "abuses" of their privilege of residence.

Tunnermann served as both ambassador to the United States and the OAS and maintained throughout that the United States had no right to expel him, citing guarantees contained in the OAS charter.