His raptor's gaze from the old photographs has softened, and the dashing, combed-back hair is now gray and thin. He may be slower today, and very fragile. But Oscar Niemeyer—who turns 100 years old on Dec. 15—climbs the stairs every morning for a full day of work in his airy, sunny penthouse office in Rio de Janeiro, overlooking Sugar Loaf Mountain and Copacabana Beach. He keeps his light, well-pressed tropical cotton shirts open at the collar and remains very much the Latin gentleman, down to his suspenders and impeccable leather shoes.

Perhaps the last surviving pioneer of early modernism, Niemeyer, who worked with Le Corbusier in the 1940s on the design for the U.N. headquarters in New York, still has more than a dozen large-scale reasons for getting up each day. The architect of Brasília continues to add monumental buildings to the federal mall of Brazil's capital—a new hemispherical national museum, the sinewy national library next door, and a sprawling, low-rise rectory adjacent to his famous crown-of-thorns Cathedral of Brasília. Closer to home, on the serpentine shores of Niterói island, just opposite Rio, the 1988 Pritzker laureate has designed a cultural acropolis along the waterfront that includes a civic theater with an undulating roof, a monumental Catholic cathedral and equally large Baptist church, and the Niemeyer Museum, its hemispherical form split open at the entrance.

Further down Niterói's shore road—named after the architect himself—he recently finished a ferry terminal and the Museum of Contemporary Art, a futuristic building that rises from its podium like a chalice toasting the 360-degree panorama. As in Brasília, the monumentalized buildings are surrounded by open space, and each is designed in striking forms distinct from the others. A sketch in ceramic tile on the theater's façade acts as a Rosetta stone explaining the design, its undulating roof lines derived from the mountains, ocean waves, and sea nymphs he has drawn.

Niemeyer commands the celebrity of a rock star in Brazil and is being honored this year with exhibitions and publications. But he remains modest. “I just did my work,” he says. “I tried to do the things I liked to do, lighter things, while working freely and always exploring technology, especially concrete, with all its sculptural potential. I think architecture finally is personal, a matter of intuition and invention. Each architect has to find his own architecture.”

The day I visit, Niemeyer works in his office on conceptual sketches before passing them along to associates. Within arm's reach are tightly shelved rows of books: Rilke, Camus, Lenin, Sartre. “I spent my whole life at the drafting boards, but I think the human questions—the problems we face, lessons of philosophy—are more important. Architecture starts in the mind,” he says. “Even if I did my work with great care, what's important is to protest against human injustice. When you see suffering all around you, it is hard to think architecture is fundamental. But it can help and complement more important things.”