An article in yesterday's editions incorrectly reported the starting time and place for a demonstration against the Reagan administration and the U.S. military presence in El Salvador. The protest march, expected to end at the Mall entrance to the Pentagon, will start at 11 a.m. at the State Department, at 23rd St. and Constitution Ave.

In the first major political street demonstration against the three-month-old Reagan administration, thousands of dissidents are expected to march on the Pentagon tomorrow to protest the U.S. military presence in El Salvador and what march organizers say is an accelerating U.S. war buildup at the cost of domestic social services.

The turnout could be blunted, some organizers fear, because of internal squabbling between the two coalitions sponsoring the protest, but a spokesman for one of the groups, the Peoples' Anti-War Mobilization, said they are expecting "tens of thousands" of people and at least 500 buses (25,000 passengers fully loaded) are lined up from out of town. Police officials say organizers told them initially to expect 3,000 to 10,000, but said the number could grow.

With confirmed groups coming from as far away as New York and Detroit, the protesters will march from the Lincoln Memorial at 1 p.m. across Memorial Bridge and on to the mall entrance of the Pentagon for an afternoon of speeches and entertainment.

Two other smaller and unrelated demonstrations are also scheduled here today. This morning, the D.C. Coalition Against Reagan-Barry Budget Cuts, a group supported by several local labor unions, will march from the District Building to the White House and back. This afternoon, the Progressive Labor Party, a Communist group, will observe its annual May Day celebration with a march from Meridian Hill Park to the White House.

Reminiscent of Vietnam antiwar rallies a decade ago, Sunday's mass protest will include labor, religious, antidraft and traditional peace groups, but organizers also hope to bring in contingents of blacks, Hispanics and homosexuals, all largely absent from the antiwar movement of the past.

The shape, direction and constituency of Sunday's demonstration, in fact, have developed slowly and painfully since last winter with widely varying factions, ranging from liberals, pacificists and democratic socialists to flinty Marxist splinter groups, bickering over control of the demonstration, its location and even the date it was to be held -- all also reminiscent of Vietnam-era organizing efforts.

The result has been creation of the two sponsoring coalitions, the Peoples' Anti-War Mobilization (PAW) and the Ad Hoc May 3 Unity Committee, each with its own separate constituent members.

Critics in the May 3 Unity Committee say PAM is dominated by the Workers World Party (WWP), a splinter Trotskyist group, and its youth affiliate, Youth Against War and Fascism. PAM acknowledges it has WWP members but says they share control of the coalition with a wide variety of black, Hispanic and other groups.

But one Unity Committee supporter said, "We wanted a broader-based coalition" to include more traditional peace and religious groups such as SANE and the American Friends Service Committee. After protracted wrangling between the two groups, a so-called unified steering committee ended up with representatives from both the Unity Committee (including SANE, Fellowship of Reconciliation and others) and from PAM (including The National Black United Front and several other ethnic and sexual groupings).