Diogo Freitas do Amaral, deputy prime minister of Portugal's outgoing government, resigned unexpectedly as leader of the conservative Christian Democrat Party tonight in a move that could provoke a breakup of the ruling coalition and early general elections.

Freitas do Amaral was the only remaining figure of authority in the fragile "Democratic Alliance" of Social Democrats and Christian Democrats, and his loss was a serious blow as the coalition struggled to agree on a successor to Prime Minister Francisco Pinto Balsemao.

The party leader made clear that he was stepping down because of escalating internal struggles within the alliance and because he disagreed with the choice of former education minister Vitor Pereira Crespo, a Social Democrat, as Balsemao's successor.

The Christian Democrats postponed until Thursday their decision on whether to ratify Balsemao's choice of Crespo as premier-designate, but unofficial reports said that the party's political committee had resolved to press for approval despite Freitas do Amaral's opposition.

President Antonio Ramalho Eanes, a staunch political opponent of the coalition, warned yesterday that he would dissolve parliament and call an early elections if it did not come forward quickly with a convincing and durable alternative to the Balsemao administration.