A bipartisan coalition of 24 congressmen yesterday urged Interior Secretary James Watt to "preserve the sanctity of the Vietnam Veterans Memorial" by moving a planned July 2 antiwar demonstration at least two blocks away from the memorial.

"While this nation must and will uphold the right of free speech, we believe that the effect of this national demonstration held in the area surrounding the Vietnam Veterans Memorial will be to mock and degrade the patriotic service of the men and women who served in Vietnam," the congressmen said in a petition.

An Interior spokesman said late yesterday that Watt had not yet seen the petition and declined comment. A spokesman for the National Park Service, which issued a permit for the protest, said there would be no reason under park service regulations for changing the site.

Two demonstrations are planned July 2 near the memorial. The Ad Hoc Committee for a July 2 Emergency Mobilization--a coalition of antiwar veterans, liberal groups, community organizations and labor unions--applied in April and received a permit to demonstrate against the Reagan administration's involvement in Central America.

The group is scheduled to gather several hundred feet from the V-shaped granite memorial at 21st Street and Constitution Avenue NW.

A counter-demonstration organized last week by the Captive Nations Vigil Committee is scheduled for a site near 23rd and Constitution. The Captive Nations group, a broad coalition of conservative organizations, supports Reagan's policy of intervening in Central America to stop the spread of communism.

In a statement announcing the petition, Rep. Chris Smith (R-N.J.), a member of the House Veterans Affairs Committee, called the Ad Hoc Committee protest "an obscene gesture of disrespect" and commended the Captive Nations group.