The Denver City and County Building was evacuated Thursday after a bomb threat, and a search of the structure was under way, police said.

Police spokesman Sonny Jackson said the evacuation was ordered at 8:30 a.m. It wasn't immediately clear how the threat was made or how many people were in the building.

The sprawling, four-story structure houses courtrooms, City Council chambers, the mayor's office and other city offices.

Some trials were under way, and all the judges and jurors were escorted safely from the building, courts spokesman Robert McCallum said.

McCallum said he did not know how many trials were in progress. He said they would resume when the evacuation ended.

Police closed Bannock Street in front of the building, and dozens of people waited on the lawn. A handful of uniformed police officers and squad cars were visible.

The stately stone building, fronted by columns and a massive stairway, faces the state Capitol across the expansive Civic Center park.