Police spokesman Masoud Mwinyi said today that most of those arrested were scrutinised by security agencies and released, but that 447 are being held in custody under anti-terrorism laws that allow police to hold suspects longer than 24 hours.
Human rights activists have criticised the operation, saying only Somalis are being targeted. Rights groups are also concerned about police treatment of suspects.
Kenya has suffered a wave of recent explosive and gun attacks blamed on a Somali militant group. Al-Shabab militants have vowed to carry out terror attacks in Kenya to avenge the presence of the Kenyan military in Somalia fighting the insurgents.