MANAGUA, NICARAGUA, DEC. 15 -- Opposition political parties today suspended indefinitely a "national dialogue" with the Sandinista government after rejecting as inadequate a government response to their proposals for constitutional reform.

The suspension of the negotiations, which began Oct. 4 under the auspices of a Central American peace plan, came after President Daniel Ortega and his brother, Defense Minister Humberto Ortega, leveled harsh criticism and warnings at conservative opposition groups following sensitive charges in Washington by a senior military defector. The 14 opposition parties and factions participating in the national dialogue had warned earlier that they would withdraw if their reform proposals were not accepted.

The suspension appeared to deal a setback to the Central American peace plan, which already has been rocked by the disclosure over the weekend of long-term plans to build up the Sandinista military by arming up to 600,000 Nicaraguans by 1995, ostensibly to combat an eventual U.S. invasion. Most of the new troops would be members of the reserves or lightly armed local militias.

In a press conference this afternoon, opposition parties participating in the dialogue accused the Sandinistas of failing to live up to the peace accord and condemned "grave threats" against opposition leaders by the Ortega brothers.

"This indicates that the problem of war will become greater for the Nicaraguan people," said Luis Sanchez Sancho, head of the Socialist Party of Nicaragua. "We hope the government will reflect and give the Nicaraguan people and {the peace accord} a positive answer before Dec. 21."

Opposition leaders are demanding positive action on their proposed constitutional reforms by that date, the last day of the current National Assembly session. Otherwise, opposition sources said, the reforms could not take effect, under existing procedures for modifying the constitution, until 1989.

On Nov. 24, 14 opposition parties handed presidential representative Carlos Nunez a list of 17 constitutional reforms they said they wanted to "democratize" the constitution and comply with the peace agreement. Most of the 14 are conservative opposition parties, but the group also includes the Communist Party of Nicaragua and Sanchez's party both of which generally criticize the Sandinistas as "bourgeois." Besides the ruling Sandinista National Liberation Front, only the Marxist-Leninist Popular Action Movement opted not to support the proposed reforms.

Among the reforms are measures that would ban presidential reelection, family succession to the presidency and voting by members of the military. Other points would limit presidential power, establish a politically nonpartisan armed forces, guarantee an independent judiciary and define the separation of the military, the government and political parties.

In essence, the reforms would change the fundamental nature of the political-military system the Sandinistas have built here.

"To maintain this structure is antidemocratic and against" the regional peace plan, opposition politician Erick Ramirez said today in reference to the Central American peace accord.

Nunez, the Sandinista chairman of the national dialogue, said President Ortega declines to take a stand for or against the measures. Instead, Nunez said, he would recommend that they be taken up in the National Assembly, where the Sandinista front has a two-thirds majority.

Opposition leaders said their move was not a definitive breaking off of the dialogue but a "suspension" in hopes that the government will show more flexibility on the proposals.