Nowhere is the shock greater than in Siena. For many here, Monte dei Paschi is more than a bank. It is “Babbo Monte,” or Daddy Monte, the city’s largest employer and greatest patron. For as long as anyone can remember, its money has helped pay for charities and civic works, including Siena’s signature annual event, the colorful Palio horse races around the Piazza del Campo each summer. Indeed, the bank’s largest shareholder, the charitable Monte dei Paschi Foundation, has long operated as a sort of shadow government here.

Now, everyone wonders what will happen without Babbo Monte’s money.

“Nothing falls from the sky anymore,” said Mario Marzucchi, president of Misericordia di Siena, which provides health care to the city’s poor and operates a fleet of ambulances. The charity is struggling to maintain its services and to renovate its clinic, located in a former Benedictine monastery.

Caterina Barbetti, president of a cooperative that operates nursery schools, said she had been forced to reduce free child care for the city’s poor. She used to depend on Babbo Monte, too. “Now,” she said, “he has left.”

Monte dei Paschi has occupied its palace in Siena’s old town since the bank was founded, although it has added modern trappings like bulletproof glass doors. The bank’s archives are in a vaulted room once used to store weapons. In the piazza out front stands a statue of Sallustio Bandini, an 18th-century Tuscan economist who was an early advocate of free trade.

Where Monte dei Paschi goes from here will be determined largely by its chairman, Alessandro Profumo, a prominent banking executive brought in from Milan. Mr. Profumo, 56, is no stranger to controversy. In 2010, he was ousted as chief executive of another Italian bank, UniCredit, after the Libyan government acquired a large stake in that bank.