Libyan Oil Minister Abdessalam Zaggar said here today that Libya plans to take its case directly to the American people through the news media to show that the Reagan administration is "unfair" in portraying it as a terrorist nation.

Speaking to reporters here, the Libyan official said: "We do believe the American people have the right to listen to every opinion," and challenged the administration to bring before the public its evidence of an alleged Libyan plot to assassinate President Reagan.

"It's a very dangerous thing to say a country sent some people to kill a president without having the evidence," he told a press conference following a meeting of the 13-nation Organization of Petroleum Exporting Countries (OPEC). "The international opinion didn't see it. I did not see it. They didn't show it."

Zaggar said that if U.S. officials had caught some individuals who planned to kill Reagan, Libya had nothing to do with it.

"The American people might kill President Reagan, and some of them tried it without Libya," he said. "Did Libya conspire in 1963 to kill President Kennedy? It did not. If someone is nuts and wants to kill the president, they can do it in the United States. No problems."

Zaggar stressed repeatedly during his press conference that Libya had no quarrel with the American people, who he said had believed in the principles of freedom since the time of President Lincoln and "the freedom fighters" of the American independent struggle against the British.

The Libyan oil minister was clearly angry at the lack of support shown here for Libya by OPEC. Libya asked the organization yesterday to adopt a plan of joint action against the United States in retaliation for the administration's recall of American oilmen working in the North African country, and the decision of several companies to close down their operations there.

He blamed Saudi Oil Minister Sheik Ahmed Zaki Yamani for the OPEC refusal to take Libya's side publicly against the United States and said the organization had supported Iraq, Algeria and even his own country in earlier feuds with Western oil companies.

"We in Libya are being persecuted by a superpower and it was the national duty of our Arab brothers to support us ," Zaggar said. "But our Saudi brother, Sheik Yamani, stood against us. I'm very sorry to have to say that."

He said Libya was prepared to fight like a "cornered cat" against the United States and had already received private assurances of assistance from other countries, including Eastern Bloc ones, to keep the Libyan oil fields running.

He also indicated Libya intended to rely primarily on its own efforts and experts to run any oil field abandoned by U.S. companies.