The authorities in Yemen say they have begun deporting a number of foreign Muslims who had been studying at religious schools.
A police official said more than 100 students had been arrested on immigration charges, and all would be expelled.
The students included nationals from Britain, France, Indonesia, Malaysia, and Sudan.
Correspondents said the operation is part of efforts by Yemen to rid itself of a reputation as a haven for Islamic extremists.
It follows speculation that Yemen could be a target in what the United States calls the second phase of its war on global terrorism.
In recent months Yemeni security forces have launched a series of missions to hunt for suspected members of Osama Bin Laden's Al-Qaeda network.