"We met with a political rep. in the embassy, Greg Legrefo, and talked about the dire situation in Gaza and international complicity for more than hour .... but the bottom line is the US supports the siege of Gaza.
"The US Army Corps of Engineers is even providing technical assistance to build an underground wall [to stop the Gaza tunnel networks from operating]."
An impromptu demonstration, reported on the Twitter micro-blogging service, began as soon as police prevented the groups progess on a side street near the embassy, keeping them there for hours.
"We believe the US Embassy asked Egyptian State Security to act against its own citizens and prevent them from entering the Embassy," Gael Murphy, one of the activists, said.
"We are outraged as US citizens about being detained simply for trying to get to our embassy," she said.
US embassy spokesmen could not immediately be reached for comment.
Organisers of the Gaza Freedom March say 1,300 people from around the world came to Egypt to try to enter Gaza to deliver aid and to participate in a peaceful march protesting the closure of the Gaza Strip's borders.
Since the activists were told last month that they would not be able to march on Gaza, they have staged a series of small protests around Cairo.
US citizen Hedy Epstein, an 85-year-old Holocaust survivor, on Monday said she would go on hunger strike to protest Egypt's refusal to allow the march to proceed.