As a jet plane roared over the Lincoln Memorial, Joan Baez was singing "Blowin' in the Wind." She lifted her head toward the sky, "Sing it, Joan," she sang out to herself. "Overpower the noise."

Spread out before her, last night's crowd of 10,000 stretched along both sides of the reflecting pool, listening to the veteran activist make "a plea, not a protest" for the boat people of Southeast Asia.

Every few minutes throughout Baez's hour-long appeal in song, another jet thundered toward a landing at National.

"It's the airlift," she said hopefully, gesturing at one of the planes.

Earlier in the evening, Vivaldi's "Four Seasons" welcomed the gathering crowd over huge speakers bunched on the steps descending toward the pool.

Fran Oglesby, 19, dangling her painted feet, popped the cork on a bottle of champagne.

"I have great respect for Joan Baez and what she's doing for peace. I'm concerned about the boat people. I'd like to go to her school for nonviolence."

Above her, inside the roped-off stage area, civil rights leader Bayard Rustin, elegant in a checked gray suit and curved cane, listened patiently to the reading of a petition.

"I don't care who else signed it. I'll sign it," he told the young man offering the document. "I'd have preferred that they didn't mention, Guam, but that's good enough."

"Thanks" replied the young man, "I'll put your name on it."

"They originally wanted to do it up there," said Lt. Gary Treon of the D.C. Police, motioning toward the steps closest to the Memorial. "We told them they couldn't interfere with the people from all over the nation who are visiting the monument."

Nearby, San Cao, a Vietnamese student leader from Columbia University, helped his friend Dat Tran attach a sign to the ropes.

"Before our country fell to the communists," said Tran, "she helped our enemies. Now she helps us. We hope Jane Fonda will change her mind too."

Told about the remark later, Rustin didn't agree.

"Joan has never taken anything but a human rights position. She's right to hold to her position. And thank God she is."

As Rustin walked off to take his seat near the head of the Buddhist Congregation of Washington, to whom Baez offered her first greeting as she took the stage, groups began to hoist their placards and makeshift cloth signs.

"The Sea Should Not Be a Tomb," read one.

"Help Vietnamese Refugees to Help Themselves," asked another, in bright red letters across a white sheet.

"Send those, the homeless, tempest tossed, to me...," began a third, decorated with a small drawing of the Statue of Liberty.

Not far away, Rep. Paul Simon (D-Ill.) talked about congressional action for the boat people.

"The real issue is not to continue to pick up boat people, but to stop the exodus. We have to do what is presently politically unpopular. We have to sit down with the Vietnamese government and discuss things."

On a grassy slope by the pool, surrounded by picnickers, Bob Vins, a 20-year-old Marine, sat pensively on a blanket.

"I like the music, but I'm not much into politics."

Asked about Baez's anti-war activism over the years, he shrugged.

"I don't know much about it."

As concert time moved closer, the cameramen, photographers and reporters began to jostle one another for space near the mikes. One cameraman was working for Baez.

"Her organization asked us to record the event. I don't know what they're going to use it for."

Pat Barso, Baez's, road manager, said it was for history.

Baez understood the importance of the other jostlers, too.

"Try to be patient with the cameramen," she told the crowd in an aside between songs. "We need them."

Glowing under the lights in a billowly purple blouse and sleek black pants slit up to the top of her thighs, Baez delivered her message without hesitation.

"We're here to show that we're human, that we have hearts. And that not one more person should die at sea if we have anything to say about it.

"My God," she exclaimed. "There is no excuse for it. We have the equipment, so let's save them."

"Thank you, Joan," yelled Hoa Nguyen, a young Vietnamese refugee who came here four years ago.

As the blue-helmeted policemen, their backs to Baez kept the fringes of the stage clear, she ran through her protest-song repertoire, inserting new lyrics for the boat people.

"There grows a tree in paradise," she chanted, "that the refugees call the tree of life."

Then, and in repeated asides, she called for the president to "send out the Seventh Fleet."

"I tried to get a meeting with President Carter today," she joked, "but as you know, this is a very difficult time of year."

A march to Lafayette Park after the concert had been planned days in advance. Just before the candlelight procession began, Baez asked the audience to join her in John Lennon's "Imagine."

"You may say that I'm a dreamer," she prompted the crowd, "but I'm not the only one." CAPTION: Picture, Joan Baez at the Lincoln Memorial, by James M. Thresher