FRANKFURT, West Germany (AP) _ Americans and Germans demonstrated Friday at the former Nazi death camps of Dachau and Bergen-Belsen, and Jews from around the world prepared to protest President Reagan's plan to visit the Bitburg military cemetery.

Two U.S. Jewish leaders led a group of 80 Americans and Germans to the former Dachau concentration camp near Munich for a memorial service. Among the Americans in the group were feminist author Betty Friedan and comedian-turned- activist Dick Gregory.

Reagan and Chancellor Helmut Kohl of West Germany will travel to Bergen- Belsen and then Bitburg Sunday as part of the ceremonies marking the 40th anniversary of the surrender of Nazi Germany on May 8, 1945.

Reagan and Kohl have said they are going to Bergen-Belsen to commemorate victims of the Holocaust, and to Bitburg to demonstrate ''the miracle of reconciliation between former enemies.''

However, many groups have criticized the Bitburg visit because 49 members of the Nazi's Waffen SS are among the 2,000 German soldiers buried there. The Waffen SS was the combat arm of the organization, but the SS also guarded concentration camps where millions of Jews were killed during World War II.

An estimated 50,000 people, most of them Jews, died at Bergen-Belsen.

Henry Siegman, executive director of the American Jewish Congress, said during the service at Dachau: ''Mr. president, Mr. chancellor, even at this late hour we ask you do not dishonor the martyrs. Do not go.''

The group at Dachau also paid tribute to two students, brother and sister Hans and Sophie Scholl, who were beheaded in 1943 for leading an anti-Nazi resistance group known as the ''White Rose.''

At Bergen-Belsen in northeastern Germany, rabbis Avraham Weiss and Ronald Schwartzberg of the Hebrew Institute of Riverdale, N.Y., and three West German students continued a prayer protest.

Police have told them they must leave Saturday night, but Weiss said, ''We have no intention of leaving Saturday evening.''

About 500 students from more than a dozen nations, including Israel, the United States, Canada and European countries, will take part in a protest Sunday at Bitburg, Dave McKowsky, chairman of the Jerusalem-based World Union of Jewish Students, said in an interview in Frankfurt.

Moshe Ronen, president of the North American Jewish Students' Network, said the students gathering in Frankfurt for protests do not oppose German-American reconcilation, but ''there is no reconciliation with the SS.''

''I can't understand how he (Reagan) can go to Bergen-Belsen and within the same hour go to Bitburg,'' Ronen said. He and McKowsky flew to Frankfurt this week to organize the protest.

Fifteen New York state legislators arrived in Luxembourg, just across the border from Bitburg, and said they would stage a protest Sunday near the cemetery.

''We are going to Bitburg because we want the world to know that Reagan's actions do not represent the feeling of most of the American people,'' said spokesman Bill Feingold.

In Jerusalem, an American Jewish leader said U.S. authorities in West Germany are preventing a demonstration during Reagan's visit to Bergen-Belsen.

Menachem Z. Rosensaft, chairman of the International Network of Children of Jewish Holocaust Survivors, said he was told his organization could not get acces to the former Nazi camp from Saturday through Reagan's visit on Sunday.

''They are able to achieve at Bergen-Belsen what they would not be able to do within the United States, which is to stop an orderly protest near the president while he is engaging in a public function,'' Rosensaft, a New York City lawyer, told a news conference.

He said he planned to lead a delegation of 50 people representing the 5,000 U.S. members of his group, to say prayers at the camp after Reagan leaves.