In the witness box Babar avoided looking at the defendants
Wearing a plain grey pullover, Mohammed Babar sat in silence in the witness box, staring straight ahead, awaiting the arrival of the jury.
It was already late in the afternoon with most of the day lost to legal argument of one kind or another and the sense of anticipation in the packed courtroom was palpable.
Babar, a 31-year-old college drop-out from the US, was to be the key prosecution witness in the trial.
He had already pleaded guilty to terrorist-related offences in New York. Now he was in London, about to give evidence against a group of men accused of plotting bomb attacks in Britain.
A door opened and the jury filed in. This was only their third day in court, but Babar was the first witness to appear in a trial due to last several months.
Not once did Mohammed Babar acknowledge the men claimed by the prosecution to be his former associates
The main body of the court was packed with lawyers, some of them barely visible behind stacks of files and papers. And in the tightly-packed rows of seats allocated to the press, not a single chair was empty.
In the dock, some of the defendants glanced up at the public gallery where their friends and relatives were watching.
After the initial drama of the opening prosecution speech that had occupied the first two days, everyone knew that the trial proper began now, with the appearance of the first live witness.
But not once did Mohammed Babar acknowledge the men claimed by the prosecution to be his former associates.
Though they were only a few metres from him, flanked by prison officers, he ignored them, looking only at the jury box as he began to answer questions from David Waters, QC, the principal lawyer for the Crown.
A university drop-out, he described how he had become radicalised as a Muslim activist while doing a number of unskilled jobs
A large man, with broad shoulders, Babar stood to swear the oath, then sat down again in the tiny wood-panelled witness box. Craning their necks, the journalists behind him saw that he had a beard and wore metal-framed glasses. Little else was visible.
He spoke softly, with a clear American accent, explaining that he had moved to the US with his family from Pakistan at the age of two.
A university drop-out, he described how he had become radicalised as a Muslim activist while doing a number of unskilled jobs.
He explained that even though his mother had survived the al-Qaeda suicide attacks on the World Trade Center, he decided soon afterwards to travel to Pakistan to fight what he called the "jihad" in neighbouring Afghanistan.
Tomorrow he would be back. And for many more days
Some of the jury took notes as Babar explained that he thought it was his responsibility to fight for his "brother" Muslims, even if it involved taking up arms against American soldiers.
And he gave the names of a number of fellow militants he had met in Pakistan, among them some of the defendants in this trial.
The prosecution had already admitted that Babar was an accomplice in the alleged plot - someone whose motives might be questioned, perhaps as an attempt to secure a lighter sentence in the US.
After little more than half an hour, the day's proceedings were over.
By the time the court was cleared and the public gallery emptied, a police helicopter was already circling overhead and an armed convoy was assembling in the streets outside the Old Bailey.
The heavy gates suddenly opened and with a squeal of sirens he was gone.
But tomorrow he would be back. And for many more days, as the panoply of defence lawyers prepared their cross-examination of the American "supergrass" so central to this English terrorist trial.