A new version of the 1960s peace rally returned to a Washington church last night as speakers from the worlds of entertainment and foreign affairs conducted a "town meeting" that climaxed with pop singer Jackson Browne, accompanied by a computer-driven synthesizer, singing, "By the dawn's early light, by all I know is right, we are going to reap what we have sown."

The town meeting at All Souls Church on 16th Street NW, one of several around the country sponsored by a Los Angeles-based group called the Committee of Concern for Central America, leveled broadsides against Reagan administration policies in El Salvador and Nicaragua.

Before a crowd of about 800, actress Susan Anspach sang a song, M*A*SH* actor Mike Farrell delivered a ringing peroration mocking George Bush's "kick some ass" remark, and actress Diane Ladd decried what she said were incidents of torture in El Salvador.

Wayne S. Smith, former chief of the U.S. interest section in Havana, who resigned in protest in 1982, called on the nation to "live up to our ideals.

"And how do we do that when we're printing manuals telling people how to assassinate other people?" he said, referring to a CIA-sponsored primer for rebels in Nicaragua. " . . . This is not what the United States is supposed to stand for. We have to do something about it."

Former CIA analyst David MacMichael said of the downing Friday in El Salvador of a plane carrying CIA employes reportedly conducting surveillance: "I guess the agency will keep trying to find one shipment of arms. Even if it kills them."