President Reagan, striking back at Managua's Sandinista government for "what they did to us," said Tuesday he has ordered the expulsion of Nicaraguan Ambassador Carlos Tunnermann and seven other diplomats.

Reagan disclosed the retaliatory move at a picture-taking session in the Oval Office, saying, "I have told the State Department to send their ambassador and seven comrades back to Nicaragua."The president indicated he does not intend to severe diplomatic relations with Managua but said that move "is always a possibility."

The Nicaraguan government had ordered the expulsion of U.S. Ambassador Richard Melton and seven other Americans Monday for "shamelessly interfering" in Nicaraguan affairs.

White House spokesman Marlin Fitzwater said that Tunnermann and seven other envoys, not yet identified, were under orders to leave the United States in 72 hours - the same amount the Americans in Nicaragua were allowed to wind up their affairs.

The president, who has harshly assailed the Marxist-led Sandinistas and supported the Contra rebels efforts to oust the government, noted that the tit for tat moves do not cut off relations altogether.

"There are some people that were not ejected that are still there and the same would be true here," he explained.

"In other words," Reagan said, "we're going to return the favor. We're going to do to them what they did to us. The other (severing relations) remains always an option."

Secretary of State George Shultz had described Monday's expulsion orders against the American as an "absolute outrage."

The surprise moves against the Americans - coupled with a new crackdown on internal opposition - appeared part of an orchestrated offensive by the ruling Sandinista front to gain ground against two of its three biggest enemies - the United States, the domestic opposition and the Contras.

Foreign Minister Miguel D'Escoto had summoned Ambassador Melton to the Foreign Ministry in Managua Monday afternoon to tell him the government was declaring him and seven other U.S. diplomats "persona non grata."