Ousted Panamanian President Eric Arturo Delvalle appeared in public Friday for the first time since he went into hiding in his homeland and said President Reagan assured him of "his commitment to democracy and civilian rule" in Panama.

"It's really been a nightmare," said Delvalle, 51, Panama's constitutional president who was removed from office Feb. 26 after trying to fire strong-man Gen. Manuel Antonio Noriega as Panama's military chief."We have a real problem in the middle, in the heart of America, and I believe it will be a definite blow to democracy, freedom and civil and human rights in all of the Americas if we continue to have a regime of that sort in Panama."

But Delvalle, flanked by Sen. Alfonse D'Amato, R-N.Y., as well as Panama's ousted ambassador to the United States and a dozen bodyguards, told reporters in Manhattan he has not ended his fight to topple Noriega.

"I want to make it clear that upon finalizing my conversations here I am returning to my country, to Panama, to stay there and fight until Panama and its people regain civil and democratic rule as has been our custom and tradition for many, many years," he said.

"I'm going to repeat some words of Mark Twain, who said, `The reports of my demise are premature,' " Del-valle said.

Delvalle said he was cheered by what he called "an extended phone call with President Reagan" Friday morning.

"I was heartened by this phone call from the president," Delvalle said. "He reassured me of his support and his commitment to democracy and civilian rule in all our countries and very specially in Panama."

Delvalle, speaking in fluent English he learned as an undergraduate at Louisiana State University, declined to say whether Reagan had promised him more concrete measures but said weekend strategy meetings with administration officials are planned.

The White House did not comment immediately on the conversation.

Delvalle also declined to outline what he called his "agenda" for restoration of Panamanian democracy.

"You'll see. Just let me do it," he said with a smile.

Reagan recognizes Delvalle as Panama's president and calls Noriega a "dictator." In response to Delvalle's ouster, Reagan imposed emergency sanctions April 8, barring U.S. companies and individuals from making any payments to Panama's government and precipitating an economic crisis there.

The political crisis began in February when federal grand juries in Miami and Tampa, Fla., indicted Noriega on charges of helping Colombian cocaine traffickers.