Government officials said today that thousands of Nicaraguans had responded to the declaration of a state of emergency by volunteering to serve in the "popular militias."

The 30-day state of emergency was announced yesterday by Daniel Ortega, coordinator of the ruling junta, following the bombing of two bridges in northern Nicaragua Sunday and press reports from Washington last week that the Reagan administration had approved a covert action plan to weaken the Sandinista revolutionary government.

Ortega, one of nine Sandinista commanders who rule the country, said the Central Intelligence Agency was behind the destruction of the bridges, reiterating his government's charges that the United States is seeking to overthrow it.

A Nicaraguan Embassy spokesman in Washington pointed to six incidents of successful or unsuccessful sabotage against Nicaraguan airplanes or economic targets since mid-December.

Under the state of emergency all individual rights and guarantees are suspended, and a number of security measures have been imposed such as censorship of newspapers and radio news shows and restrictions on travel within the country.

Ortega said in his speech that the government would strictly control the news media. Minutes before he spoke, the Interior Ministry announced the shutdown of Radio Catolica, a station run by the Roman Catholic Church, for violating a stringent two-year-old communications law.

A communique said the station was closed indefinitely because it reported the dynamiting of the bridges before the government announced it.

The ruling Sandinista National Liberation Front controls or heavily influences two of the country's three newspapers, the only television network and most radio news shows. The daily newspaper La Prensa and a few radio news shows take a strong opposition stand, despite government restrictions.

Ortega did not give further details on what the suspension of rights entailed. Government sources said it affected freedom of expression, political and union activities and the right of habeas corpus. Normally, the government can hold people without formal charges for only three days, but under the state of emergency they can be held indefinitely.

The United States has charged that Nicaragua's efforts to build up its military and the militias poses a threat to its neighbors in Central America. Nicaragua maintains that the military buildup is necessary to defend the country against "counterrevolutionary" exiles seeking to overthrow the Sandinista government.

(n Washington, State Department spokesman Dean Fischer said the United States is concerned about the suspension of guarantees and feared that it might be "a prelude to a general crackdown on those democratic elements which remain in the country."

[Fischer declined to comment on Nicaraguan charges that the state of emergency was required to protect the country from U.S. covert action. The Washington Post reported March 10 that President Reagan had approved a plan to build a paramilitary force of Latin Americans to launch attacks against vital targets such as bridges and power stations inside Nicaragua.]

[Fischer persisted in his refusal to discuss that aspect of the situation, despite suggestions from reporters that his silence could be taken as a confirmation of the charges about covert action. "We just don't address allegations of this sort," he insisted.]

[In Miami, Pedro Ortega, leader of an anti-Sandinista military force called the National Liberation Army, said his men inside Nicaragua had blown up six bridges, including the two mentioned in Nicaraguan government statements yesterday.]