The U.S. Embassy was sprayed with bullets this morning in an incident that Acting Ambassador Frederic L. Chapin said had "all the hallmarks" of right-wing terrorism directed by Salvadoran ex-major Roberto D'Aubuisson.

There were no injuries and minimal damage but as public fears of a rightist coup attempt mounted, Chapin used the occasion to make his first unequivocal public statement of support for the current coalition government headed by Christian Democrat Jose Napoleon Duarte.

Duarte, meanwhile, ruled out a trip to Europe to meet with lefist leaders, weclomed promises of U.S. military as well as economic aid, and declared that the government sought the arrest of D'Aubuisson.

"The people on the extreme right are always there trying to pull a coup d'etat -- permanently -- and we are fighting against them," he said. Reacting to criticisms yesterday by D'aubuisson, Duarte added, "If we can find him, we will arrest him" for incitement to a coup.

The president denied D'Aubuisson's charge that the Christian Democrats, whom he had called the right wing of the Communist Party, would sell out the country to the Communists. D'Aubuisson also said he believed the Reagan administration would "favor" the ouster of the Chrisian Democrats.

Chapin set out to refute any such notion. "This [shooting] incident has all the hallmarks of a D'aubuisson operation. Let me state to you that we oppose coups and we have no intention of being intimidated," Chapin said.

The recently arrived diplomat had been instructed to lower the profile of the U.S. Embassy after the sometimes flamboyant performance of dismissed ambassador Robert White. At today's press conference, however, Chapin said Washington supports this government's fight against lefist guerrillas, its implementation of reforms and its movement toward national elections.

Then, in a statement received from Washington late this afternoon, Chapin announced that the U.S. government supports President Duarte and his government "and we oppose a coup or anyone who seeks to change the Duarte government."

There has been considerable concern both here and in Washington that the Reagan administration was not making its support of the current regime clear to the right-wing extremists of El Salvador.

Even as today's shooting took place, Duarte was addressing the nation and the international press in an effort to undercut right-wing criticism.

Duarte said he would not go to Europe to international Christian Democratic meetings where attempts were being made to initiate negotiations between Duarte and the lefist guerrillas supported by European Socialists.

Newspapers here carried stories this morning quoting unidentified Salvadoran political sources who called such negotiations a "dangerous maneuver." Conservatives here have publicly expressed their belief that the guerrillas are attempting with the aid of the Socialist International to gain through political means what they have failed to win militarily.

Asked about the risks to his government of entering negotiations with the guerrillas or their political representatives, Duarte said:

"In every move we make there is a risk. So this [decision not to go to Europe for the moment] is another proof of how we're moving right in the middle."

Duarte said he is open to dialogue with the left but would not negotiate a single position in the government even as a prelude to national elections, which he has said are vital to a political solution here.

Duarte said he recognizes the need for extensive U.S. military aid to fight the guerrillas and reports that he thought economic aid more important and necessary are untrue. He said that economic, military, social and political aid are all necessary for the development of a democracy here.

He emphasized the role of the private sector here, saying his party is not the "enemy" of private enterprise and telling his countrymen the $300 million he hopes to get from U.S. and other sources will be used mainly to pay for imports that benefit the private sector.

To stop fears that a major crackdown by the military would follow the end of an amnesty offered to repentant guerrillas, Duarte announced the extension of the amnesty for another 30 days. It had been scheduled to end March 11.

Yesterday, after months in hiding, D'aubuisson resurfaced at a secret meeting with the press and said he had consulted with Reagan's advisers and associates and believed Washington would favor the ouster of the Christian Democrats to be replaced by an entirely military regime.

D'Aubuisson, who denies any terrorist connections, has been blamed by both the Christian Democrats and the U.S. Embassy for assassinations and at least two coup attempts in the last year. A charismatic leader with close ties to the most conservative elements of Salvadoran society, including former colleagues in the armed forces, D'Aubuisson was arrested briefly last May.

Wealthy Salvadorans picketed the residence of then-ambassador White, thereatening to keep him confined until D'Aubuisson was released. When White broke out with the aid of Marine guards after three days, the U.S. Embassy was blasted by gunmen in apparent retaliation. They shot from a passing pickup truck using heavy-caliber automatic rifles.

Today at 11 a.m., a similar gray truck with four men in back once again passed by the embassy and at least five shots were fired at the banker-like structure from what appear to have been 7.62 mm military-type rifles. Fence posts were bent by the impact of the bullets and a window of the cashier's office was broken.

Past attacks by leftist insurgents have been with Molotov cocktails and, once, with Chinese-made rocket-propelled grenades. No serious injuries were suffered in any of the assaults.