NORFOLK, Va. (AP) _ The Soviet Union's top military officer got a first-hand look at American military power as he watched the Navy flex its sea and air muscle and saw Marines deploy men and equipment under simulated combat conditions.

It was part of a whirlwind tour of American military bases Thursday by Marshal Sergei Akhromeyev, chief of the Soviet Union's general staff.

After watching Air Force demonstrations at nearby Langley Air Force Base today, the group was to leave for visits to bases in Texas, Oklahoma, and South Dakota. They will return to Washington, D.C., on Sunday.

Akhromeyev, a soldier since 1940 when he joined the Soviet army as a 17- year-old, started the tour Thursday at Camp Lejeune, N.C., where elements of the 2nd Marine Division demonstrated quick deployment. In the afternoon, the party was flown out to the aircraft carrier Theodore Roosevelt as it cruised about 80 miles off the Virginia coast.

There the commander-in-chief of the Atlantic Fleet, Adm. Frank B. Kelso II, gave the Soviet officer a tour of the Navy's newest nuclear-powered carrier.

The carrier visit featured a demonstration of various aircraft aboard the ship, including the F-14 Tomcat and F-18 Hornet, and was topped by a ''wall of water'' demonstration in which an A-6E Intruder attack bomber dropped 18,000 pounds of bombs, creating a huge surge of water.

Asked whether the United States should be showing its best military hardware to a potential enemy, Kelso said: ''I guess there's always a risk that you give something away when you show anybody anything. But remember, deterrence also is a perception, and it's hard to have a perception unless you can see what you want to think about.''

Akhromeyev said he was pleased with the demonstration.

''I liked it very much as a military man,'' he said through one of his aides. ''I observed the up-to-date weapons and the highest standards of performance, and, of course, I liked it.''

At Camp Lejeune, the Marines gave the visiting Soviets a demonstration of a Marine Air-Ground Task Force, coordinating infantry, airplanes, helicopters, artillery and other equipment.

Akhromeyev, the first Soviet military officer to visit the base, gave a ''thumbs up'' sign at a demonstration of an AV-8B Harrier, an attack plane which can take off and land vertically.

His host is Adm. William J. Crowe Jr., chairman of the Joint Chiefs of Staff. Crowe has accepted an invitation to make a reciprocal visit to the Soviet Union.

After watching the Navy's demonstration Thursday, Crowe and Akhromeyev went to colonial Williamsburg to join their wives in a carriage ride down historic Duke of Gloucester Street.