The Washington office of the American Civil Liberties Union asked a federal judge yesterday to order D.C. police to let the Christian Knights of the Ku Klux Klan march Sunday on 11 blocks of Constitution Avenue NW to the Capitol.

Judge Louis F. Oberdorfer said he would issue a decision after a hearing tomorrow.

ACLU legal director Arthur Spitzer asked the judge to order the D.C. government to issue a permit for the Klan to march on Constitution Avenue or to order the D.C. police to help U.S. Park Police protect the marchers along an alternate route on Madison Drive on the Mall.

The spokeswoman for Mayor Marion Barry and the spokesman for D.C. Police Chief Isaac Fulwood Jr. said they were unaware of the ACLU suit and could not comment.

In a letter to Spitzer on Monday, Fulwood said he denied the Klan's request for a permit to march along Constitution Avenue starting at 14th Street NW because police could not "adequately assure public safety for the length of the route proposed." Instead, Fulwood said he would approve letting the Klan follow a route that covered four blocks of Constitution Avenue.

Spitzer said Fulwood's concern for public safety was an illegitimate reason to deny the permit.

"The Klan's right to engage in a peaceful demonstration is being abridged by the threatened violence of other people opposed to them," he said, adding that is known as the "heckler's veto."

"If everyone else gets to march the route they want from the Washington Monument to the Capitol and a particular group is denied that right for illegitimate reasons, we consider that a serious violation," Spitzer said.

The All Peoples Congress, a radical leftist coalition, has vowed to stop the Klan from marching. On Labor Day weekend, D.C. police discouraged the Klan from marching on Constitution Avenue because several thousand demonstrators were massed along the route and had blocked the street.

Klan leader Virgil Griffin, of Mount Holly, N.C., said he felt his group's constitutional rights had been violated and vowed to return. At the behest of D.C. police, he changed his permit application last week to split the march between federal and District-controlled territory.

But the agreement unraveled when D.C. police told Park Police they would not "stand shoulder to shoulder" with them along the Mall route, said U.S. Park Police Chief Lynn Herring.

On Monday, Herring said he could not recommend that the permit be granted if his 500 officers were not provided assistance.

At a meeting held at U.S. Park Police headquarters yesterday, D.C. police said again they would not stand alongside Park Police if the march took place on the Mall, according to Park Police Deputy Chief Mickey McQueeney, who attended the meeting.

But D.C. police spokesman Reginald L. Smith denied last night that any official request had been made for District police to stand side-by-side with Park Police officers. "No one has made a request of us to do that," he said.

Smith said the city police force was prepared to help the Park Police on Sunday if there was "an immediate need."