An emotional Yasser Arafat said yesterday that President Reagan betrayed him and implied that the United States should be held responsible for last week's massacre of Palestinians in Beirut.

Reagan "said that the city would be safe; he guaranteed it," Arafat told a group of about 2,000 people -- including deposed Ugandan president Idi Amin -- in a courtyard at Palestine Liberation Organization headquarters here.

It was Arafat's first public appearance since the massacres at the Sabra and Shatila refugee camps last week.

"Now they are bringing back the multinational force," Arafat said, "but it's too late and I blame Reagan for this also."

Arafat said Israeli soldiers were directly involved in the killings along with major Saad Haddad's Christian militia. He sent the crowd into a frenzy when he slammed his fist to the table and said, "Major Haddad does not have bulldozers. Where did he get them from?"

The Palestinian leader said the Lebanese Army also should shoulder part of the blame for the massacre "because they took the arms from our people and left them no means to defend themselves."

In almost a whisper, he added: "I made a deal with U.S. envoy Philip C. Habib for three reasons: to protect the city Beirut , to protect the refugee camps and to release all prisoners. I was tricked.

"I could tell you more," he added, "but I can't. It's not the right time."

Arafat told his audience -- most of them Palestinians residing here -- that during the withdrawal negotiations, Israeli Prime Minister Menachem Begin sent a message to him. "Don't forget Mr. Arafat," he mimicked Begin to the crowd, "it is us that is surrounding Beirut, not you that is surrounding Tel Aviv."

At that everyone laughed. They also laughed when Arafat told them, "More than 210,000 shells fell on Beirut, two times the force of the bomb on Hiroshima, I think. Excuse me, I'm an engineer but I've forgotten engineering."