President Reagan yesterday ordered the Coast Guard to intercept on the high seas ships suspected of carrying people seeking to enter the United States illegally.

The order was aimed at Haitians. More than 60,000 Haitians are estimated to have entered the United States illegally by sea since 1972, and between 1,000 and 1,500 are arriving each month, according to White House officials.

Until now, the Coast Guard could challenge suspect ships only after they had entered U.S. territorial waters.

Reagan said he had determined that the illegal immigrants are detrimental to the interests of the United States and that they "have threatened the welfare and safety" of the communities where they land, mostly in south Florida.

Although the United States gave refuge to 125,000 Cubans who arrived in boats in 1980, neither the Carter nor Reagan administrations has agreed that the Haitians also qualify as refugees fleeing repression. Haitians, by Washington's definition, are not taking to small boats to escape the right-wing dictatorship in Haiti, but are leaving for economic reasons and therefore cannot claim refugee status. This leaves them subject to normal immigration quotas and procedures.

Some civil rights groups have challenged that distinction, claiming that Haitians are fleeing political repression of the non-communist regime of Jean-Claude (Baby Doc) Duvalier, just as certainly as Cubans flee political repression of Fidel Castro's communist dictatorship.

Reagan's executive order yesterday follows negotiations between the United States and Haiti to reach an agreement on controlling migrants. It also follows a determination by Attorney General William French Smith that the president had the power to act without seeking legislation from Congress.

"The entry of undocumented aliens from the high seas is hereby suspended and shall be prevented by the interdiction of certain vessels carrying such aliens," Reagan said in a proclamation issued late yesterday.

The president's executive order said that despite the order the United States would observe "our international obligations concerning those who genuinely flee persecution in their homeland."

The order gives the Coast Guard the authority to:

Stop and board vessels "when there is reason to believe that such vessels are engaged in the irregular transportation of persons" in violation of U.S. law or the law of any country with which the U.S. has an agreement on controlling immigration. (Haiti is the only such country, a White House official said.)

Question people on such ships, examine passengers' documents and take steps to establish the registry and destination of the ship.

Return such vessels and their passengers to the country from which they came.

A Justice Department spokesman said "the main thing we're hoping for from the new order is a psychological impact in Haiti." He said the administration hopes that would-be immigrants will be discouraged by the increased risks and decide not to chance the trip.

By making immigration riskier, the government also hopes to deter Americans whose business is smuggling immigrants. "If we make it risky enough, the people in the business will go into another business," the spokesman said.

In Miami, Ira Kurzban, a lawyer for the National Emergency Civil Liberties Union and the Haitian Refugee Center Inc., said, "I think it's impracticable, unworkable and illegal and we certainly intend to challenge it," the Associated Press reported.

Kurzban said the policy will be challenged on grounds it violates a U.N. convention and protocol on the status of refugees. That document states that refugees who claim they are persecuted in their homeland will not be returned, AP reported.

Reagan's order is part of his overall effort to control illegal immigration that brought more than 800,000 people into the United States last year.

When he announcd the overall policy July 30, the attorney general declared, "We have lost control of our borders. We have pursued unrealistic policies. We have failed to enforce our laws effectively."

"The ongoing migration of persons to the United States in violation of our laws is a serious national problem detrimental to the interests of the United States," Reagan said yesterday.

"A particularly difficult aspect of the problem is the continuing illegal migration by sea of large numbers of undocumented aliens into the southeastern United States," he added.