A New York City police detective yesterday described his experience running a confidential informant in Brooklyn and Staten Island mosques, and the limits he occasionally set on the scope of that surveillance.

This glimpse into the anti-terrorism efforts of the NYPD came during the third week of the trial of a Pakistani immigrant, Shahawar Matin Siraj, who is charged with a 2004 conspiracy to blow up the Herald Square subway station on 34th Street.

During testimony in federal court in Brooklyn, Detective Stephen Andrews spent more than three hours under cross-examination describing how he ran surveillance of the al-Noor mosque on Staten Island, and of the Islamic Society of Bay Ridge in Brooklyn in late 2003 and early 2004, more than half a year before Mr. Siraj, 23, was arrested for the abortive bomb plot.

Referring to his informant, a 50-year-old Egyptian-born American citizen, Osama Eldawoody, Mr. Andrews said: “He was supposed to be on the lookout for whatever was going on. His eyes and ears were to be open.” Mr. Eldawoody attended about 575 services as a police informant over a 13-month period.

Although he would eventually monitor the defendant, Mr. Eldawoody initially cast his net wide, befriending imams and, on one occasion, returning with the license plate numbers of several worshippers. Mr. Andrews testified that he later instructed Mr. Eldawoody to cease jotting down license plate numbers, but said he nonetheless ran the tags in a database. Mr.Andrews testified that some of the information Mr. Eldawoody gathered, including the number of worshippers present in the mosques, was needed more for bureaucratic reasons than police work.

“It was just something he needed to pass along to me to let me know he attended services,” Mr. Andrews said.

One of Mr. Siraj’s attorneys, Martin Stolar, has suggested that the police department breached rules governing its surveillance efforts when it sent Mr. Eldawoody into the mosques to take down the information of people in prayer.