CAIRO—As Egyptians prepare for massive protests against President Mohammed Morsi on Sunday, a large piece of opposition activists' anger is being directed at the U.S. and its perceived support for Egypt's ruling Islamists.

A flurry of newspaper articles, talk shows and public statements over the past few weeks have singled out the U.S. for particular scorn while accusing America's diplomatic mission in Cairo of acting as a sort of puppet master behind Mr. Morsi's administration.

Anger against the U.S. is nothing new in the Middle East, and neither are conspiracy theories in which Washington plays a strong, silent hand.

But rarely have such theories placed U.S. influence so squarely behind Islamists such as Mr. Morsi, a former leader in the powerful Muslim Brotherhood that the White House helped to subdue for decades by backing successive anti-Islamist autocrats.

The planned nationwide protests against Mr. Morsi on Sunday, organized by the secularist opposition, are aimed at deposing the president a year after he took office. On Thursday, the opposition rejected the president's offer made in a nationwide address a day earlier for dialogue on reconciliation.