Three surveillance officers, identified as 1661, 1488 and 1498, detailed how they followed the defendant, Abid Naseer, in March and April 2009 in the cities of Manchester and Liverpool in northern England.
The two men and one woman wore heavy black and dark-brown wigs and partially shielded their eyes behind spectacles. The men also wore beards.
The agents said they followed the defendant, codenamed "small panel," as part of Operation Pathway as he visited a shopping center in Manchester, allegedly the target of the plot, a mosque and other locations.
They called it one of the most serious terror plots since the 9/11 attacks on New York and Washington.
Naseer denies the charges and faces life in prison if convicted.
The surveillance officers said they followed the defendant in the company of two other men, codenamed Happy Skater and Glass Pendant.
The trial in a US federal court in Brooklyn allowed the defendant, who is representing himself in court, to cross- examine the first spy, who wore a John Lennon-style dark brown wig and thin-rimmed spectacles.
Naseer was extradited from Britain in 2013 and is charged with conspiring to use weapons of mass destruction, commit murder abroad and supporting Al-Qaeda, among other charges.
He was first arrested in 2009 in Britain with 11 other men suspected of preparing an attack against the Manchester mall.
They were released without charge, but Naseer was arrested for a second time in July 2010 at the request of Brooklyn prosecutors, who accused him of participating in the plot to attack the New York subway in 2009.
