Demonstrators, some in neatly pressed blue suits and others in punkish outfits of studded black leather, marched together to the thumping beat of drums outside the White House yesterday afternoon as a diverse parade of about 750 people turned out to protest U.S. military aid to Nicaraguan rebels.

The rally, sponsored by the Coalition to Stop the U.S. War in Nicaragua, was timed to crystallize opposition to the Reagan administration's proposal to send $100 million to Nicaraguan rebels known as contras before the House of Representatives begins debating the proposal this week, organizers said.

The protest was held under gray skies and in brisk winds.

Families came with children. Some of the protesters had gray hair, others had no hair at all, save for multicolored spikes jutting out from otherwise clean-shaven scalps. There were no incidents and no arrests, according to U.S. Park Police.

"This is one of those issues where there are only two sides," said Tomas Squip, one of the protesters. "It's brought a lot of different groups of people on the left together."

Squip, 23, said he was a singer for Beefeater, a local band that is part of the "punk underground."

In the same procession was Patricia Cullen, 57, who said she opposed aid to the contras because she is a Catholic.

"There are a lot of gray-haired ladies out protesting," said Cullen. "It's a moral issue with me. I don't want to see contra aid."

The protest had been endorsed by an estimated 60 organizations, including local chapters of the Gray Panthers, a senior citizens group, and the Democratic Socialists of America, said coalition spokeswoman Joanne Rappaport.

"It's really a broader range of groups that are involved . . . . A lot of these faces are unfamiliar," Rappaport said, surveying the picket line.

"I didn't come as part of a group. I came as just an individual," said one protester, who said he worked for the government and declined to give his name. The man, dressed in a brown tweed jacket, said this was the first protest he had attended since the Vietnam War. Memories of Vietnam also ran high for other protesters, such as Reuben Gomez, chaplain of a Veterans of Foreign Wars chapter from Santa Cruz, Calif.

"Reagan is trying to forget the lesson of Vietnam that we paid for with our blood," said Gomez. "We won't let the administration forget. We are going to be their conscience."