President Reagan signed a 25-year extension of the Voting Rights Act of 1965 yesterday in a ceremony that brought all the leaders of the major civil rights groups to the White House for the first time in his administration.

Reagan used the occasion to declare himself an unswerving defender of the right to vote--"the crown jewel of American liberties," he called it--and to attempt to diminish the significance of his widely heralded differences with civil rights groups.

"Yes, there are differences over how to attain the equality we seek for all our people," Reagan said in brief remarks before signing the bill.

"And sometimes amidst all the overblown rhetoric, the differences tend to seem bigger than they are. But actions speak louder than words. This legislation proves our unbending commitment to voting rights. It also proves that differences can be settled in the spirit of good will and good faith."

But black leaders who attended the East Room ceremony observed afterward how difficult it had been to secure Reagan's endorsement and indicated that they did not believe it signaled any significant change in Reagan's civil rights position.

NAACP president Benjamin Hooks congratulated Reagan for "belatedly" supporting the measure but said, "I don't think it indicates any change of heart at all." He added that the Justice Department has "systematically rolled back enforcement of civil rights legislation."

Operation PUSH president Jesse Jackson was even more restrained in his response. "We're glad, we celebrate the extension of the Voting Rights Act, but if it is extended and not enforced, it is merely an Indian treaty. The focus now shifts from Capitol Hill to the Justice Department," he said, and, referring to the assistant attorney general for civil rights, he wondered, "What will Brad Reynolds do now?"

Reagan's differences with the civil rights leadership have ranged across a variety of budget, tax and social policy issues.

But rights leaders, in the face of what most regarded as a virtual counter-revolution by the Reagan White House, decided early to concentrate on getting the third extension of the voting rights law passed. First enacted in the Johnson administration, it was responsible for gaining millions of blacks in the South the right to vote and, as a result, substantially rearranging the political landscape.

Last November, key presidential advisers had urged Reagan to come out for extension, but Attorney General William French Smith wanted to weaken provisions of the bill passed overwhelmingly in the House.

Reagan's initial compromise of the opposing views was to declare his support of either extension or a "modified version" of the original act. It was a fuzzy position that would become part of a lengthening indictment by civil rights leaders and advocates. In the end, however, he embraced the bill.

Attending the signing were members of Congress, the Cabinet, black and Hispanic White House aides and an array of civil rights spokesmen that included, in addition to Jackson and Hooks, Urban League president John Jacob, the Rev. Joseph Lowery, head of the Southern Christian Leadership Conference, and Coretta Scott King, widow of Dr. Martin Luther King Jr.

As a group, the black leaders and their predecessors had been familiar figures at White House meetings over the last two decades. But this was their first joint appearance at the Reagan White House, which had shunned them and tried to build up a group of academically oriented black conservatives to take their place.

The decision to invite them yesterday did not come without debate and trepidation, but White House aides said it finally was decided that they were owed that much recognition because of their role in getting the voting rights act extended.