“There’s no question that there’s a lot of pent-up demand from ordinary investors,” said Ryan Feit, the site’s chief executive and one of its founders. “At the end of the day, that means there will be more capital available for small business.”

The amount of money backers will be allowed to invest depends on their income. Those with an annual income or net worth of less than $100,000 will be allowed to invest up to $2,000 in a 12-month period, or 5 percent of the lesser of their income or net worth, whichever is greater. Those with an income and net worth of more than $100,000 will be permitted to invest up to 10 percent of the lesser of their annual income or net worth.

The equity shares they buy will be risky, illiquid investments. Investors will generally be required to hold on to the shares for at least one year, and there are not yet many marketplaces for those seeking to sell shares in private companies, which are difficult to value.

Some critics are deeply skeptical about the quality of the investments that will be available. “Ninety-nine percent of these deals will prove to be unprofitable,” said Andrew Stoltmann, a lawyer who specializes in securities fraud. “This is a disaster waiting to happen.”

Others counter that the new rules will allow entrepreneurs’ family, friends, customers and professional contacts to invest in ventures that they want to support.

“I think it’s going to really make a difference for businesses that are not especially fashionable for professional investors,” said James Dowd, the chief executive of North Capital Private Securities, a broker-dealer that focuses on private fund-raising. “They want to invest in companies that have the potential to be disruptive to an entire industry. You don’t see a lot of capital flow into ordinary consumer and retail businesses.”

The S.E.C. on Friday also proposed changes to several other fund-raising rules, including those governing intrastate offerings. More than 25 states have adopted their own crowdfunding rules to let local businesses raise money from residents within the state, often with fewer regulatory requirements than the federal rules. The commission suggested striking down a rule that blocked those intrastate offerings from being advertised to out-of-state investors — a quirk that prevented companies from publicizing their fund-raising campaigns on their own websites or on social media sites.