MANAMA, Bahrain  As more than 100,000 protesters descended into the streets on Friday, women uniformly dressed in black flowing robes carried signs saying, “Revolution: The only solution.”

Three weeks of pro-democracy protests in this island nation have followed the pattern of those in Egypt and Tunisia, with cellphones and Facebook posts propelling the movement and a botched, deadly crackdown by security forces two weeks ago serving to embolden the demonstrators.

Yet those who lead and take part in the nearly daily demonstrations here say they fear at least one key difference: The United States may not be fully on their side.

“The U.S. is not acting like they did in other countries,” said Ali Najaf, who marched on Friday amid a sea of red-and-white Bahraini flags. “We thought they would support the people.”