Voters in Catalonia, Spain's northeastern region, approved a sweeping overhaul of its constitutional relationship with the central government on Sunday, endorsing a plan to grant broad new powers of self-government.

Members of the center-right Popular Party and other critics have complained that the plan could open the way to an eventual declaration of independence by Catalonia, one of Spain's richest regions.

They also contended that the plan threatened Spain's unity because it was likely to encourage all of Spain's regions to make increasingly ambitious demands for greater autonomy.

Prime Minister Jos?uis Rodr?ez Zapatero of the center-left Socialist Party, who campaigned actively for the autonomy plan, argued it was the only effective way to keep the restive region content within Spanish borders.

He has expressed a willingness to discuss similar plans for other regions. ''With this new statute, the identity of Catalonia will be better recognized,'' he said at a news conference in Madrid after the vote. ''It will have better instruments for administering self-government, and it will preserve the rich pluralism that is inherent to Catalan society.''

With 99 percent of the votes counted, the Catalan regional government said Sunday night that 74 percent had approved the plan, while 21 percent had rejected it. The remaining votes were blank or void.

It said that turnout was about 50 percent.

The plan, which has already been approved by the national Parliament in Madrid, is scheduled to take effect on July 1.

The measure grants Catalonia more control over a variety of issues, including tax collection, immigration policy and judicial affairs. It acknowledges that Catalonia considers itself a nation, requires that residents learn the Catalan language and declares that the region's powers of self-government emanate from the people of Catalonia rather than from the Spanish Constitution.

Mariano Rajoy, the president of the Popular Party, the main opposition group in Parliament, said many of these provisions were unconstitutional and posed a direct challenge to the authority of the central government.

''We must stop this project to liquidate constitutional Spain,'' he said at a news conference here after the vote. ''We are going to use all of the political and judicial arguments that we have so that this does not go into effect.''

He said his party would file an appeal with Spain's constitutional court in hopes of blocking the plan. Constitutional experts said the appeal had little chance of succeeding.

A potent separatist movement has persisted for more than 100 years in Catalonia, a region of seven million inhabitants whose capital is Barcelona.

Most of the region's prominent politicians advocate either separating from Spain or eliminating the region's subordinate status to the central government.

But before Mr. Zapatero took office in April 2004, the region's push for more autonomy generally met resistance from Madrid.

His decision to endorse many of Catalonia's autonomy demands led to the most significant drop in his approval ratings since he became prime minister.

His decision drew criticism not only from political opponents but from members of his own party.

Jos?ono, the former defense minister, resigned in April after repeatedly denouncing the autonomy plan. Alfonso Guerra, one of the Socialist Party's most prominent elder statesmen, said the plan had helped create a political climate that was so polarized along regional lines that it reminded him of the Soviet Union at the end of the cold war.

The most dramatic reaction came from two military leaders who conjured up memories of Spain's history of military intervention in politics by suggesting in January that the armed forces were ready to act unilaterally to quell Catalan demands for more autonomy.

Both were removed from their posts.