SAN FRANCISCO — Facebook unveiled an ambitious plan on Tuesday to create an alternative financial system that relies on a cryptocurrency that the company has been secretly working on for more than a year.

The effort, announced with 27 partners as diverse as Mastercard and Uber, could face immediate skepticism from people who question the usefulness of cryptocurrencies and others who are wary of the power already accumulated by the social media company.

The cryptocurrency, called Libra, will also have to overcome concern that Facebook does not effectively protect the private information of its users — a fundamental task for a bank or anyone handling financial transactions.

But if the project, which Facebook hopes to begin next year with 100 partners, should come together, it would be the most far-reaching attempt by a mainstream company to jump into the world of cryptocurrencies, which is best known for speculative investments through digital tokens like Bitcoin and outside-the-law e-commerce, like buying drugs online.