Medea Benjamin, co-founder of CODEPINK, an anti-war group, was making her way to Gaza for a women's conference when she was detained at the Cairo airport jail without explanation, CBS News reported.
"Help. They broke my arm. Egypt police," Benjamin said on Twitter.
U.S. Embassy confirmed that it provided consular assistance to Benjamin and that she had since been deported to Turkey.
Benjamin earlier tweeted that her jail cell was cold and that she was served "dirty stale bread and dirty water."
No other details on Benjamin's arrest were released.
The Egyptian government has been cracking down on opposition members, arresting dozens of supporters of ousted President Mohamed Morsi.