Havana ? Fidel Castro on Wednesday led a massive anti-U.S. march, part of what Cuban officials described as the largest political protest in the country’s history.

Cubans poured into streets and plazas nationwide to protest U.S. policy toward Cuba and Bush administration demands that the Cuban leader abandon socialism. In all, Cuban officials said there were 845 marches and 2,330 rallies across the nation.

“The Americans are trying to impose something on us that we don’t want. Leave us alone,” said Ramon Padron, 63, a retired construction worker who joined the Havana march.

Castro loyalists and foes have very different views of why the Cuban president in the midst of an economic crisis would stage such a spectacle, one that paralyzed shops, banks, schools and public transportation.

His supporters said Castro was only trying to send a message to President Bush. “It’s a popular, broad, patriotic referendum, a demonstration of the people’s absolute support of this government,” said a Cuban official who requested anonymity.

Foes said Castro was trying to derail the Varela Project, a petition drive seeking a referendum on whether people want free enterprise and greater civil and political liberties. The project’s organizers submitted more than 11,000 signatures to Cuba’s National Assembly on May 10.

“If government leaders have so much support, why not do a referendum on the Varela Project?” asked Oswaldo Paya, a coordinator of the petition drive. “What are they afraid of?”

The first of the protests began around 7:30 a.m. State-run television broadcast images of huge crowds in Havana and in Santiago de Cuba, the second-most populous city. Castro, 75, led marchers along Havana’s waterfront, passing the U.S. Interests Section, as the American diplomatic mission is called.

U.S. officials peered down from a balcony. Some counted the protesters as they passed by, and one estimated the crowd at 486,000, well below the Cubans’ million-plus claim.

“Long live socialism, down with the lies!” yelled the Havana marchers, a blend of young and old, military and civilian, black and white.

Castro announced the protests Monday after Cuban labor, youth and other organizations said they had approved a proposal to modify the 1976 constitution. The goal: To make socialism “untouchable.”