The nation's largest black denomination unseated its conservative president of 29 years with a Louisiana pastor more inclined toward social activism.

Ignoring schism threats, some15,000 delegates voted nearly 3 to 1 to retire the Rev. Joseph H. Jackson as president of the 6.5 million-member National Baptist Convention U.S.A., and replaced him with the Rev. T.J. Jemison, a Baton Rouge, La., pastor who has been secretary of the church and who enjoyed the backing of the Rev. Jesse Jackson.

In contrast to his predecessor's unwavering, anticommunist stances reminiscent of the cold war and his coolness toward aggressive civil rights activities, Jemison ran on a platform promising "to address major issues that negatively affect black people -- unemployment, inadequate participation in the political process, and declining support for public education."

Jemison was the first challenger to Jackson since 1961 when the Rev. Gardner Taylor was defeated in an uproarious convention in Louisville, Ky., that resulted in one death. Taylor and others then pulled out of the denomination and formed the Progressive National Baptist Convention.

In his successful drive to oust Jackson Thursday, Jemison -- whose father had been president of the denomination before Jackson took over in 1953 -- promised to recommend the post of president emeritus, along with a $15,000 honorarium for Jackson, who is believed to be approaching 90.

One of the strongest supporters of the new president was the Rev. Jesse Jackson, the head of the Chicago-based PUSH, who characterized Jemison as a friend of the late Martin Luther King and a man who shares King's ideology.

In contrast, the ousted Joseph Jackson had split with King over activist tactics. He also opposed civil rights demonstrations, advocating instead job-preparedness as a better hope for black achievement.

In his "state-of-the-church" address preceding the election, Jackson praised Margaret Thatcher for what he called Britain's successful challenge of Marxism in the Falklands and described Israeli Prime Minister Menachem Begin as "one of the great saints of peace" for his victory over the "Marxist" PLO in Lebanon.

But Jackson criticized the United States for letting itself be pushed around by Cuba, Vietnam and Iran and for "conquering Israel who had conquered the PLO" by sending in American troops to escort the PLO safely out of Lebanon.