By Noah Browning

CAIRO, March 4 (Reuters) - A U.S. rights activist on Tuesday accused Egyptian police of breaking her arm after they detained her in Cairo airport while she was on her way to a humanitarian visit to neighboring Gaza.

"Help. They broke my arm. Egypt police," tweeted Medea Benjamin, founder of protest organization Code Pink, which rose to prominence for opposing the U.S. invasion of Iraq in 2003.

She said she was now in Turkey after being held in a poor conditions in a cell at Cairo airport.

"I was brutally assaulted and I am furious. I am now in a hospital in Istanbul airport," she texted Reuters after being deported.

"I think it's a fracture and a dislocated shoulder up to my neck and down the side of my body."

An Egyptian airport official denied any abuse and said she had been deported after 20 hours. The official told Reuters that Benjamin was on a watch list for "involvement in acts harmful to Egyptian national security".

The U.S. Embassy in Cairo told Reuters it knew of her detention.

A Code Pink spokesperson said Benjamin had traveled to Egypt to meet international activists and bring lanterns to the neighboring Gaza Strip, which is blockaded by Israel and Egypt and suffers chronic power shortages. The representative was not aware of the alleged abuse.

Egypt on Tuesday banned the Hamas Islamist organization which runs Gaza from operating on its soil. Egypt says the group is complicit in a wave of shootings and bombings targeting its security forces.

(Editing by Michael Georgy and Angus MacSwan)