Police Commissioner Raymond W. Kelly was at the center, his presence underscoring how the operation was fraught with challenges for the Police Department. There could be no repeat of episodes in recent weeks, like the pepper-spraying of protesters, that violated department rules and created a firestorm of public sympathy for the squatters.

And so the police operation to clear Zuccotti Park of protesters unfolded after two weeks of planning and training. Officials had prepared by watching how occupations in other cities played out. A major disaster drill was held on Randalls Island, with an eye toward Zuccotti. Officials increased so-called disorder training — counterterrorism measures that involve moving large numbers of police officers quickly — to focus on Lower Manhattan…

Police Commissioner Raymond W. Kelly was at the center, his presence underscoring how the operation was fraught with challenges for the Police Department. There could be no repeat of episodes in recent weeks, like the pepper-spraying of protesters, that violated department rules and created a firestorm of public sympathy for the squatters.

And so the police operation to clear Zuccotti Park of protesters unfolded after two weeks of planning and training. Officials had prepared by watching how occupations in other cities played out. A major disaster drill was held on Randalls Island, with an eye toward Zuccotti. Officials increased so-called disorder training — counterterrorism measures that involve moving large numbers of police officers quickly — to focus on Lower Manhattan…

Mr. Browne said about 142 people were arrested in the park. Most of the arrests were for “disorderly conduct and resisting arrest,” he said. The highest-ranking officer on the scene was Joseph J. Esposito, the chief of department, the top uniformed officer in the department. Phil T. Pulaski, the department’s chief of detectives, was also there. Mr. Kelly, while present, was “not directing operations,” Mr. Browne said.