An estimated 3,000 demonstrators, many of them wearing T-shirts declaring, "I am a contra too," and waving handmade placards marked with anticommunist slogans, gathered yesterday in Lafayette Park to show their support for President Reagan's plan to provide $100 million in aid to the Nicaraguan rebels fighting the Sandinista government.

Organizers of the rally included the American Security Council, a lobby that focuses on national security and defense issues, and Citizens for Reagan, a lobby that favors aid for the counterrevolutionaries, also known as contras.

The controversial aid package to the contras was approved by the Senate March 27, 53 to 47. The House, which had rejected an aid proposal, is scheduled to vote on it again in two weeks.

"The narrow question that will be answered is whether the United States will renew military assistance to the embattled freedom fighters of Nicaragua," said Patrick J. Buchanan, White House communications director, in a brief speech at the rally.

"But when the House votes yea or nay to military aid for the contras, it will give America's answers to other questions -- answers for which the whole world is waiting," Buchanan said.

He said the House vote will "tell us whether the contras are the next fighting ally of the U.S. to be abandoned in the field . . . whether a communist future is certain for the Nicaraguan people or whether freedom will be given a second chance."

The crowd -- which organizers said included Cuban Americans from New York, New Jersey and Miami -- cheered its approval of the remarks made by Buchanan, who spoke only in English, and other speakers, who spoke in Spanish and then repeated their remarks in English.

The bilingual speakers included Jeane Kirkpatrick, former U.S. ambassador to the United Nations; Elliot Abrams, the State Department assistant secretary for inter-American affairs, and Curtin Winsor Jr., former U.S. ambassador to Costa Rica.