clouds of smoke rose over cairo as a symbol of egypt's old regime went up in flames. it happened after an angry but peaceful protest by police officers. cnn's ivan watson takes us to the scene. >> reporter: this is the interior ministry, what has long been the headquarters of the police state that has helped keep egypt under control until revolution transformed everything in this country. moments ago, this building was in flames. a pillar of smoke that rose over downtown cairo and collapsed parts of the building. in the last few minutes we've seen military police on the ground arresting some people. also rushing away one man in an ambulance. this fire erupted within hours of a large, peaceful protest in