MANAGUA, NICARAGUA, OCT. 8 -- Nicaragua's main opposition coalition, in a procedural dispute, this morning walked out of the first working session of a dialogue between the Sandinista government and the opposition.

The dialogue, involving 12 opposition parties that range from far right to far left, was opened by the government to comply with a regional peace plan signed Aug. 7 in Guatemala. Under the pact, the government does not have to hold direct talks with the armed rebels, known as counterrevolutionaries or contras, but it must meet with unarmed opposition groups.

Carlos Huembes, president of the Democratic Coordinating Group, left the meeting after only a few minutes to press his coalition's demand to seat 14 representatives instead of the four invited by the government.

But the gesture lost some of its punch when the biggest party in the Coordinating Group, the Social Christians, stayed in the meeting.

"We don't see any reason to exclude ourselves from an encounter that could shape the history of our country," said Miguel Angel Manzanares, a Social Christian participant at the talks. Another important opposition party that is not a Coordinating Group member, the Independent Liberal Party, also stayed in.

The Democratic Coordinating Group unites 14 opposition groups and parties to the right of the Sandinistas, including labor unions and business organizations.

The government invited the Coordinating Group to send four representatives, one for the alliance and three for the legally registered parties participating in it. The Coordinating Group wants to send one representative for each member organization. Other opposition politicians pointed out that could provide an opening for the ruling Sandinista National Liberation Front to overwhelm the meeting with delegates from its dozens of unions and professional groups.

Carlos Nunez, the Sandinista head of the National Assembly who is the government's delegate to the dialogue, said U.S. Embassy officials had urged the Coordinating Group to withdraw from the dialogue.

Huembes said the alliance was only suspending its role and may return.

Two previous efforts at national dialogue since 1984 ended quickly in failure.

According to Social Christian leaders, 18 party members who were taken off a bus and drafted into the Sandinista Army as they were returning home from a rally Sept. 27 have been released through the intervention of Ortega.