The Rev. Jesse L. Jackson stood at Martin Luther King Jr.'s old pulpit today and, in the rousing rhythms of a master preacher, warned a church full of the faithful that the memory of the murdered civil-rights leader is being distorted into that of a "nonthreatening dreamer."

He also lambasted President Reagan as a hypocrite for his visit this morning to a Washington elementary school for a commemoration of what would have been King's 57th birthday.

As the setting sun gilded the stained glass of Ebenezer Baptist Church, Jackson reminded the crowd that, in life, King "called himself a drum major for justice. In death, however, he is projected as a dreamer . . . a nonthreatening dreamer."

King, who on Monday becomes the first black honored with a federal holiday, "was not a Pied Piper, a kind of free-spirited man walking along with a dove in one ear and an American flag in the other . . . about five feet removed from reality," Jackson went on.

Voices in the congregation called out, "Tell it, Jesse!" and "That's right."

"That so-called 'I have a dream speech' . . . was not a speech about dreamers and dreaming. It was a speech describing nightmare conditions," Jackson said.

He brought the group to its feet when he said, "Dr. King was not assassinated for dreaming."

King's birthday should be celebrated the way he spent his last Jan. 15 on earth, Jackson said, recalling that he had been with King that day. After eating at home with his family, King came to the Ebenezer church basement and organized the 1968 Poor People's Campaign, Jackson said.

To mark the holiday, Jackson urged boycotts, demonstrations, increased voter registration and other activities.

Jackson's drumbeat gospel contrasted with the more soothing harmonies about "bridging gaps" and "setting aside differences" that have characterized the King birthday week celebrations here.

King, Jackson said, mopping his brow as his voice rose, said to America, "You don't believe it, but might is not right. Right is might . . . . He was just a preacher. Didn't understand big banks . . . just a preacher. Never stood over a military budget . . . just a preacher. Never went up to the U.N."

Reagan, he went on, was right to call for a fight against Libyan-backed terrorism. "He was right, but wonder why nobody responded?"

"He's over the world's largest economy . . . considered the single most powerful man in the world. But when you stand before the U.N. and don't mention South Africa, when you support terrorism in South Africa and practice terrorism in Central America -- by the time you get to the Middle East, you have lost your moral authority!"

That brought another standing ovation, and Jackson continued, saying King "had a power that belongs to all of us . . . . Dr. King had moral authority. That's what my grandmother got."

Loss of moral authority was at the root, he said, of "babies making babies, drug addiction" and other problems plaguing the black community.

He reserved his harshest words for the Reagan administration, its policies on affirmative action and other issues and the president in particular.

"Today, President Reagan went over to a school and held up little black children in his hands, rubbed his eyes to look tearful," Jackson said, although witnesses at the Southeast Washington school said the president did not lift any of the children.

"The same man who didn't support the boycott in Montgomery," Jackson said, referring to Reagan. "He was old enough. Same man. Same man who didn't support the march to Selma. Same man who implied that Dr. King was a communist. The same man!"

The press, Jackson said, "will give him a free ride and project him as a kindly old man."

But he added, bringing the crowd roaring to its feet, "With all of his power . . . even America will never have a national holiday named after Ronald Reagan!"

Jackson was introduced by former Atlanta mayor Maynard Jackson, now an investment banker, one of several speakers who sounded a note of frustration about the floundering of the civil-rights movement.

He quoted a complaint from a colleague, a white businessman in New York who accused the black community of "going to sleep" and expecting to be handed jobs and economic power without demanding them. "It's the gospel truth," Maynard Jackson said. "If we don't take care of ourselves, if we don't fight for us, nobody will."

Gloria E.A. Toote, former assistant secretary of the Department of Housing and Urban Development, now a lawyer, urged blacks to "learn the mechanics of economic power," to be their own catalyst for change, to use the power of the black dollar to help other blacks