Plans for two opposing groups to hold the first mass demonstrations near the Vietnam Veterans Memorial on July 2 sparked bitter opposition yesterday from veterans groups protesting the use of the memorial as a political forum.

The planned demonstrations, focusing on the Reagan administration's growing military involvement in Central America, are also likely to prompt renewed debate about the controversial design of the black granite Vietnam memorial, according to Jan C. Scruggs, the president and founder of the Vietnam Veterans Memorial Fund.

"I think both groups should be ashamed of what they are doing," said Scruggs, who added that a broad cross-section of Americans "from George McGovern to Gen. William Westmoreland" have supported the memorial and tried to prevent it from becoming a political battleground "for the extreme right and the extreme left."

National Park Service officials said yesterday they expect both July 2 demonstrations to be peaceful, but Scruggs said that many Vietnam veterans are angry about the protests and "a lot of Vietnam vets plan to come July 2 to keep their memorial from being damaged by lunatic fringe elements."

Yesterday, the New York-based "Ad Hoc Committee for July 2 Emergency Mobilization," a coalition of antiwar veterans, community organizations, labor unions and liberal groups claiming the support of 500 groups in 87 cities, announced plans for "tens of thousands" of demonstrators to mass near the memorial to protest Reagan policies which it said would lead the United States into "another Vietnam."

A counter-demonstration hastily organized last week by the "Captive Nations Vigil Committee," consisting of a wide cross-section of pro-Reagan forces supporting U.S. intervention abroad to fight communism, is planned for a site near Constitution Avenue and 23rd Street NW, roughly two blocks from the antiwar site at Constitution Gardens, near 21st Street, where the memorial is located.

Demonstrations are banned at the Vietnam memorial, like other memorials, but are permitted several hundred feet away, said park service spokeswoman Sandra Alley. To minimize noise and distraction at the memorial, both groups will not be allowed to point loudspeakers toward the site, she said.

The Vietnam Veterans of America, calling the demonstrations "highly inappropriate," yesterday issued a statement urging all veterans to stay away July 2.

Robert O. Muller, executive director of VVA, said the memorial is a place for honoring the 57,939 dead whose names are inscribed there, "and not a site for groups or individuals who agree or disagree with American foreign policy."

In a more strongly worded statement, the Veterans of Foreign Wars called the antiwar protest by the Committee for July 2 "a mass obscenity . . . by a coalition of far left activists." VFW's national commander-in-chief James R. Currieo said: "The cynical political use of the Vietnam Veterans Memorial area by aging anti-Vietnam war activists must not be allowed to succeed."

A spokesman for the antiwar coalition, Bob Lamoth, said yesterday "We feel that using the memorial is appropriate, and we don't mean to offend anyone. But we think it is important to point out there were over 57,000 Americans killed, and the purpose of our demonstration is to prevent that from ever happening again."

A spokesman for the Captive Nations group, Amy Moritz, said yesterday that conservative groups and veterans organizations felt that the "leftist demonstration" planned for July 2 had to be "neutralized." She said groups including Young Americans for Freedom, the National Conservative Political Action Committee and others are joined in supporting Reagan's drive to stop the spread of communism.

Scruggs said he regrets that the protests are renewing the description of the V-shaped memorial as "a wailing wall" for the left and the right: "Like I say, we don't want Jane Fonda one day and Jerry Falwell the next."