The Reagan administration yesterday went on heightened alert against new Libyan terrorism, and U.S. officials said they were particularly concerned that reprisal attacks might be directed against "soft targets" such as American businesses and tourists in foreign countries.

Federal agencies charged with countering threats in this country -- the FBI, the Secret Service, the Immigration and Naturalization Service and the Customs Service -- said they were continuing the stepped-up security measures that have been in effect for months, including a watch on the 3,271 Libyan nationals in this country.

However, most officials, echoing the views they expressed after the Gulf of Sidra confrontation last month, said that any new terrorism almost certainly will occur overseas. They added, though, that the extraordinary degree of security now in effect at U.S. embassies and other overseas government installations requires an assumption that the chances of staging successful actions will be greater against so-called "civilian targets."

The Federal Aviation Administration advised U.S. airlines and airports "to be extra vigilant," and said it is requiring American airlines that fly to foreign airports to institute "special additional security measures." The FAA declined to describe these measures.

State Department spokesman Bernard Kalb said the administration does not believe that new travel advisories are required at this time. But he added: "Americans should obviously exercise caution in all international travel during this period of tension . . . . The threat exists."

Another department official, however, speaking on condition he not be identified, warned: "This summer is going to be an open season on Americans in Europe." In particular, he said, there is concern that scores of small independent terrorist groups such as radical Palestinians could heed Libyan leader Muammar Qaddafi's exhortations to go hunting for Americans.

According to the official, this concern was presented to the White House before President Reagan ordered Monday's air strikes against Libya, but in the end it was outweighed by the feeling of senior administration officials that strong action had to be taken against Qaddafi.

Other officials, working in the antiterrorism area, agreed that civilian targets overseas are of great concern. But speculation about what Qaddafi or certain terrorist groups might do, one official said, is "purely a subjective call."

In justifying the U.S. bombing as a self-defense measure to deter Qaddafi from future attacks, Reagan and Secretary of State George P. Shultz said Monday night that they had evidence of Libyan plans to attack Americans and 30 U.S. installations, including embassies.

Officials said yesterday that the allegedly targeted embassies are primarily in Western Europe, but that some are in the Middle East or Africa.

"There has been a battening down of the hatches at these embassies and others that might normally prove to be tempting targets, and we are reasonably confident that they now have been secured against bombings or attacks," one State Department official said.

Officials of federal security agencies in the United States said that so many precautions are in effect that no dramatic new measures are contemplated. The Libyan community in this country has been under surveillance since 1981, when the administration said it had reason to believe that Qaddafi was sending a team of assassins here.

Although the Federal Bureau of Investigation refuses to discuss its operations, it maintains a constant watch, aided by electronic surveillance, on the Libyan mission at the United Nations and places such as a McLean residence that houses several Libyan students.

Verne Jervis, a spokesman for the Immigration and Naturalization Service, said that 3,271 Libyans, about half of them students, are thought to be in this country on a temporary basis. He said that for several months, immigration officials have subjected Libyans entering and leaving the country to special scrutiny that includes questioning about their activities and destinations and examination of their documents.