A group of Jehovah's Witnesses missionaries, including at least six from the United States, were detained in Managua yesterday as "suspected counterrevolutionaries," U.S. and Nicaraguan officials reported.

Ten of the detained missionaries were later expelled and flew to Panama, U.S. and Nicaraguan officials said. In Panama, an airport official said later that five of the arriving missionaries were Americans.

The whereabouts of the missionaries remaining in Nicaragua were not known.

A Nicaraguan official in New York said six of the missionaries were U.S. citizens and four were Canadians. State Department spokesman Joe Reap said a total of 18 were detained, of whom 10 were American and the rest Canadian, British and West German.

The Associated Press reported from Managua that nine missionaries, five of them Americans, were seized at a house 12 miles southeast of the Nicaraguan capital.

The Nicaraguan official said the Jehovah's Witnesses were detained because they were suspected of participating in counterrevolutionary activities. The official said they had been "agitating against the government from the pulpit."

The Nicaraguan official said an official explanation would be issued today.

The arrests came as the Reagan administration is stepping up its criticism of Nicaragua's revolutionary Sandinista government for restricting political freedom and mistreating the country's Miskito Indians.

At the same time there has been a renewal of diplomatic activity to ease the growing conflict in Central America. The United States and Mexico agreed last week on a plan to seek new contacts with Nicaragua and Cuba in an effort to defuse the rising tension in the region, and Nicaraguan officials have met with the presidents of Cuba, Mexico and Panama. An informed source said yesterday that the Nicaraguan ambassador to the United Nations, Javier Chamorro Mora, had met with U.S. Ambassador Jeane J. Kirkpatrick, who also met with a Nicaraguan official last week.

The Associated Press reported the following from Managua:

The government said a Nicaraguan Moravian pastor, under arrest for "counterrevolutionary activities," was killed when he tried to escape from his cell in Puerto Cabezas in northeast Nicaragua.

A man who identified himself as Vern McDaniel of Omaha, Neb., said that the missionaries had been led off with suitcases in hand.

"I'm told there are agents now at my house. My wife and I are going there now. I suppose they are going to arrest us too," he said.

McDaniel said the arrests took place about 10 a.m. EST.

McDaniel said those arrested were: Reiner and Jean Thompson of California; Kevin and Ruby Block, he of Minnesota and she of Canada; Ken and Sharon Brian of Canada; Elfride Urban of West Germany; Margaret Foster of Iowa, and Ian Hunter of Scotland.

The Nicaraguan pastor who was killed was identified as Olfilario Teofilo Larus of Vuakil.

The pastor's village is in the region where the government has relocated 8,000 Miskito Indians amid growing discontent over the government's policies.