Cairo, Egypt (CNN) -- At least two humanitarian workers were detained by police in Egypt on Thursday after military police took over Cairo's Hisham Mubarak Law Center, Amnesty International said.

A member of Amnesty's staff, a delegate from Human Rights Watch and others were taken to an unknown location in Cairo, Amnesty said in a statement.

"We call for the immediate and safe release of our colleagues and others with them who should be able to monitor the human rights situation in Egypt at this crucial time without fear of harassment or detention," said Salil Shetty, secretary general of Amnesty International.

The group called on U.S. President Barack Obama to also push for their release.