The Rev. Jesse L. Jackson said yesterday that he has not decided whether to run for president, defusing speculation that his long-awaited announcement might come during a major black political gathering here this weekend.

He said the fact that he is still considering his decision should warn "those who would try to cast cold water on this idea or break our spirit" that his effort has not lost momentum.

"The interest is growing," he said.

Jackson, in town for the opening of the 13th annual Congressional Black Caucus legislative weekend, told reporters that on Monday he would ask for a leave of absence as president of Operation PUSH (People United to Serve Humanity), his Chicago-based civil rights and economic development organization, to devote full time to making his decision.

Associates of the 41-year-old minister and civil rights leader said they expect the decision to come within two weeks, although Jackson refused to offer any time frame.

He has returned from a European tour and said he is seeking a meeting with President Reagan to discuss U.S. nuclear weapons policy in Europe. White House spokesman Larry Speakes said he had not seen Jackson's request for a meeting. As Jackson was making his announcement at a news conference at the Shiloh Baptist Church, black politicians from around the nation were gathering at the Washington Hilton Hotel for the legislative weekend.

One of the major concerns of many is Jackson's possible presidential candidacy, which in the past several months has spurred black voter registration, underscored the significance of the black vote and divided black politicians and civil rights leaders from New York to California.

Several of the 21-member Congressional Black Caucus, all of whom are Democrats, oppose Jackson's potential bid for the Democratic nomination because they fear he would drain votes from a liberal candidate such as former vice president Walter F. Mondale, who has a better chance of winning the nomination, or one like Sen. John Glenn (D-Ohio), who they think has a better chance of beating Reagan.

In addition, some caucus members are rivals of Jackson as the premier black political leader and black political power broker.

Jackson aides say they expect some informal efforts this weekend to dissuade him from running. Jackson said late yesterday that numerous meetings he has scheduled this weekend, some presumably with elected officials who are still withholding vital support, would be important to his decision, but not final. "It will be important because the attempt to unify the masses and build a consensus among leadership is an important part of our mission," he said.

"There is a convergence of leadership here," he added, "but there are about 26 million blacks not here and a lot of other people, too."

At yesterday's news conference, Jackson said that although he has been encouraged by the emotional response to his possible candidacy, he still has not determined if he can raise enough money and develop an adequate campaign organization to "prevail" in a number of selected Democratic primaries.

He also is concerned about his safety, he said, the effect the campaign would have on his family and on Operation PUSH.

Jackson also reiterated that one of his fundamental concerns is to force a debate by the major candidates on key issues that are crucial to blacks and poor people and to make sure that their votes are not taken for granted.

Jackson returned Sunday from an eight-day visit to England, the Netherlands, West Germany and West Berlin. He said yesterday that the trip gave him a better sense of America's options in Europe and concluded that negotiation with the Soviets was preferable to further deployment of nuclear weapons in Europe.