Over the past 40 years, Mr. Leal, the official historian of Havana, has pulled off a most unusual feat. While much of Cuba’s infrastructure has crumbled and its economy has limped along, he has rebuilt and refurbished more than 300 landmark buildings in Old Havana, from fortresses built in the colonial days to famous nightspots and hotels of the city’s swinging era just before the Cuban revolution.

The center of the city was once a dark warren of cobblestone streets, worn facades and decaying ruins. Now it has some streets that rival Prague or Paris for cleanliness and beauty. Tourists throng the Plaza del Catedral, with its 259-year-old cathedral, and wander up Calle Obispo, a street lined with luxury shops, to the Floridita, the plush bar where Hemingway drank mojitos and daiquiris.

“There were years when not everyone believed in this,” Mr. Leal said, as he walked up Calle Obispo and shook hands with well-wishers. “Years when there was a lot of work, a lot of difficulties, but now it’s easier, because now you can see all the people, how they support you, they give you a smile, some happiness, and this makes it possible to continue at least for a little while longer.”

Yet the renovation has only gone so far, and tens of thousands of people are still trapped in squalid buildings just blocks from the refurbished zones, giving rise to grumbling among some residents that the renovation amounts to a Potemkin village for visitors.