LONDON — Julian Assange, the founder of WikiLeaks, said on Monday that his Web site could be forced to shut down by the end of the year because a 10-month-old “financial blockade” had sharply reduced the donations on which it depends.

Calling the blockade a “dangerous, oppressive and undemocratic” attack led by the United States, Mr. Assange said at a news conference here that it had deprived his organization of “tens of millions of dollars,” and warned, “If WikiLeaks does not find a way to remove this blockade, we will not be able to continue by the turn of the new year.”

Since the end of 2010, financial intermediaries, including Visa, MasterCard, PayPal and Western Union, have refused to allow donations to WikiLeaks to flow through their systems, he said, blocking “95 percent” of the Web site’s revenue and leaving it to operate on its cash reserves for the past 10 months. An aide said that WikiLeaks was now receiving less than $10,000 a month in donations.

Mr. Assange said WikiLeaks had been forced to halt work on the processing of tens of thousands of secret documents that it had received, and to turn its attention instead to lawsuits it had filed in the United States, Australia, Scandinavian countries and elsewhere, as well as to a formal petition to the European Commission to try to restore donors’ ability to send it money through normal channels.