President Reagan came to the site of the former Nazi concentration camp here today and declared that present generations had learned a lasting lesson from the Nazi era. Death had ruled at Bergen-Belsen, he said, but man's spirit could never be extinguished.

"And that is why we are here today," the president declared.

For many of those who took part, protected from anti-Reagan demonstrators by a vast security cordon, the power of the language seemed out of proportion in a place now so unremarkable to see.

Rising like a watchtower above the site of the former death camp -- now a clearing in the center of a forest and dotted with dozens of anonymous mass graves -- was a 100-foot television antenna constructed to beam Reagan's speech back to the United States.

By contrast with celebrations last year marking the 40th anniversary of the D-Day landings in Normandy, today's ceremony seemed stripped of a vital element -- the witnesses to the suffering that took place here. Survivors of the Holocaust boycotted the event in protest against Reagan's decision to lay a wreath at a German war cemetery in Bitburg.

Officially invited guests -- mostly local dignitaries -- were outnumbered by journalists and photographers who were themselves outnumbered by hundreds of plainclothed and uniformed security men patrolling the grounds of the former concentration camp. Jewish groups opposed to the Bitburg visit and other demonstrators were prevented from getting anywhere near the site.

When Reagan joined other western leaders on Utah Beach last year, many of the veterans there to relive the most searing experience of their lives complained that their anniversary had been preempted by the politicians.

But at least the veterans were there, and they gave the services of remembrance a link with the past that some observers felt was noticeably lacking in today's ceremonies.

Instead, the absence of former camp inmates seemed to give the ceremony today a surrealistic feeling, emphasizing the sense of isolation of the president and the media. It was as if all were enclosed not simply in a physical vacuum but in a historical vacuum, far removed from the genocide that took place here 40 years ago.

The Nazi machinery of death, carefully preserved as a ghastly lesson for posterity at other camps such as Auschwitz and Dachau, is just a memory at Bergen-Belsen. Sanitary conditions were so appalling at the camp, with bodies simply stacked in piles, that the buildings were burned after the war to prevent epidemics from spreading throughout northern Germany.

When British troops liberated Bergen-Belsen on April 15, 1945, they found about 40,000 emaciated survivors, of whom 13,000 were to die of disease in the next two months. Deaths from extermination, hunger and epidemics had been running at more than 4,000 a week in the final months before the camp's liberation.

A group of Jewish activists who staged a sit-in last week at the camp's documentation office in protest against Reagan's visit to Bitburg cemetery were persuaded to leave before the president's arrival. They returned to the camp after the president and his party were whisked away in three helicopters.