Secretary of State Alexander M. Haig Jr. is to leave today on a four-day trip to Europe aimed at countering what the Reagan administration sees as important inroads made by the Soviet Union on public opinion in Western Europe, especially in West Germany.

Officials say the general view in the administration is that the Soviets have been skillfully exploiting a variety of European concerns, especially over such war-and-peace issues as neutron weapons and U.S. plans to station new missiles on European soil, and that Moscow's efforts are having at least some adverse effects on the psychological cohesion of the Atlantic alliance.

Haig's itinerary takes him first to Spain, for what initially had been scheduled as a refueling and rest stop before official visits to Belgrade on Saturday, West Berlin on Sunday and Bonn on Monday.

But the stop in Spain has been extended for several hours, officials said, because Haig is expected to meet with the Saudi Arabian deputy prime minister, Prince Fahd. The meeting has not been announced officially.

Fahd is a brother of King Khalid, and is widely regarded as the key foreign policy force within the Saudi regime. Sources here said Haig will brief the prince on the just-completed visit of Israeli Prime Minister Menachem Begin to Washington and on the status of the controversial proposal to sell five AWACS radar planes to Saudi Arabia. That proposal will be debated in Congress soon. Haig also is expected to discuss possible Saudi involvement in speeding up the peace process in the Middle East.

But the main point of Haig's trip, officials say, will be a speech in West Berlin intended as the kickoff in a campaign to seize the public affairs initiative from Moscow and try to convince increasingly vocal critics that it is Moscow, not Washington, that is endangering peace in Europe.

In recent months there have been growing signs of anti-American attitudes in Western Europe, especially in West Germany, this country's most important military and economic ally within the 15-nation North Atlantic Treaty Organization.

While these attitudes are coming from a relatively small segment of the population, primarily on the left, the number of critics is growing, and includes church groups and many other more middle-of-the-road elements opposed to new U.S. military policies and the administration's harsh anti-Soviet rhetoric.

How much of this is home-grown fear that the United States may be spoiling for a fight that would take place in Europe, and how much of this opposition is stirred up by Moscow is open to question.

But the feeling in Washington is that the West generally has let the Soviets play too easily on European fears without counterattacking. Officials here say the West generally has failed to exploit the worth of its democratic values, ideals and free society, in comparison with Soviet-style rule, in the battle for public opinion.

Thus, Haig's speech is being billed here as an attempt to "make an inspirational pitch" to the Europeans to remind them of some basics and to help European leaders tackle those same problems.

Washington reportedly took the initiative in arranging the Haig visit to West Germany, and the choice of Berlin as the location for his speech was a deliberate attempt to dramatize East-West ideological and sociological differences.

The Berlin Wall that has sealed off the communist zone from the western sectors of the divided city was built 20 years ago last month. It has been 10 years since the United States, France, Britain and the Soviet Union signed a four-power agreement governing western access to the city.

Aside from Haig's visit and speech, the campaign to rebuild more traditional postwar images of the good guys and bad guys will include release of previously classified allied information on the extent of the Soviet military buildup in Eastern Europe.

American officials are hoping that this combination of a reminder about western values and free societies and reminders about the Soviet military threat will have some positive effect on European public opinion, though there is not great confidence evident among some U.S. officials about this.

Some sources suggest that Haig's meeting in Bonn with Chancellor Helmut Schmidt and Foreign Minister Hans-Dietrich Genscher also may touch on the public relations problems that are growing out of official rhetoric on both sides of the Atlantic.

The Europeans, fearing that the United States may push the Soviets too hard, are concerned about the extremely tough talk aimed toward Moscow by President Reagan. The Americans, on the other hand, don't like Schmidt's using high U.S. interest rates as a way to rationalize cuts in Bonn's budget, which also could fan anti-American sentiment in Germany.